Light scanning apparatus, image forming apparatus equipped with such light scanning apparatus, and control method or image forming method for such image forming apparatus

ABSTRACT

An image forming apparatus, includes: a latent image carrier whose surface includes an effective image region spanning across a predetermined width in a main scanning direction and is driven in a sub scanning direction approximately orthogonal to the main scanning direction; a latent image former which has a light source and a deflection mirror oscillating, and deflects a light beam from the light source using the deflection mirror so as to scan the effective image region with the deflected light beam; and a scanning mode controller which switches selectively between a single-side scanning mode and a double-side scanning mode, the single-side scanning mode being a mode in which the light beam is scanned only in a first direction included in the main scanning direction, the double-side scanning mode being a mode in which the light beam is scanned in both the first direction and a second direction opposite to the first direction, wherein a condition to form latent images on the latent image carrier in the single-side scanning mode is different from a condition to form latent images on the latent image carrier in the double-side scanning mode.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 11/442,736,filed on May 26, 2006, the entire contents of which are incorporatedherein by reference. The disclosure of Japanese Patent Applicationsenumerated below including specification, drawings and claims isincorporated herein by reference in its entirety:

No. 2005-158465 filed May 31, 2005;

No. 2005-161342 filed Jun. 1, 2005;

No. 2005-175111 filed Jun. 15, 2005; and

No. 2005-175110 filed Jun. 15, 2005.

BACKGROUND

1. Technical Field

The present invention relates to an image forming apparatus and an imageforming method according to which a light beam is irradiated upon alatent image carrier, which is driven in a sub scanning direction, in amain scanning direction which is approximately orthogonal to the subscanning direction to thereby form a latent image.

2. Related Art

An image forming apparatus of this type comprises a photosensitivemember, an exposure unit and a developer unit, and forms a latent imageon the photosensitive member in the following manner. In short, a lightsource of the exposure unit is controlled based on image data whichrepresents a toner image, and a deflector of the exposure unit makes alight beam from the light source scan in the main scanning direction,thereby forming a latent image corresponding to the image data on thephotosensitive member. This latent image is then developed with tonerand a toner image is formed.

In a light scanning apparatus of this type, a deflector makes a lightbeam from a light source scan on a surface to be scanned in the mainscanning direction. Further, an image forming apparatus using a lightscanning apparatus of this type comprises a photosensitive member and adeveloper unit, and the light scanning apparatus forms a toner image onthe photosensitive member. That is, a light source of the light scanningapparatus is controlled based on image data which represents a tonerimage, thereby modulating the light beam, and as the deflector deflectsthe modulated light beam, a beam spot scans over the surface of thephotosensitive member and a latent image corresponding to the image datais formed. This latent image is then developed with toner and a tonerimage is formed.

A solution proposed so far for size reduction and speed improvement of adeflector is use of a deflection mirror surface which oscillates andaccordingly serves as the deflector. In other words, in this apparatus,a deflection mirror supported at a torsion bar oscillates, the lightsource irradiates the light beam upon the deflection mirror, and thelight beam scans on the surface of the photosensitive member back andforth. JP A-2002-182147 is an example of related art.

SUMMARY

The image forming apparatus like this selectively switches between adouble-side scanning mode in which the light beam from the light sourcescans over the photosensitive member in both forward and backward scandirection and a single-side scanning mode in which the light beam scanseither forward or backward, thus making it possible to form an image ina printing mode. When a high resolution is not necessary for example, abeam spot may scan the photosensitive member in either forward orbackward, thereby forming an image at a low resolution, whereas when ahigh resolution is required, a beam spot may scan on the surface of thephotosensitive member in both forward and backward, thereby enhancingthe resolution. In addition, this image forming apparatus, reproducingtone, prints not only a line image consisting a text or the like but agradation image such as a photograph.

However, in the case of such a structure as that described above capableof switching between the double-side scanning mode and the single-sidescanning mode, development of a latent image formed in the double-sidescanning mode on a photosensitive member accompanies a problem that morethan necessary toner adheres to the surface of the photosensitive memberand an image is impaired. The reason will now be described in details.For forming a continuous line in the sub scanning direction without abreak in the single-side scanning mode, it is necessary that the beamwidth in the sub scanning direction on the surface of the photosensitivemember is equal to or wider than at least the scanning pitch in the subscanning direction in the single-side scanning mode. This is becausewhen the beam width in the sub scanning direction on the surface of thephotosensitive member is narrower than the scanning pitch in the subscanning direction, latent images formed by means of light beamirradiation do not become continuous with each other between adjacentscanning lines and a continuous line is not therefore formed. The widthof the light beam in the sub scanning direction on the surface of thephotosensitive member thus must be equal to or wider than at least thescanning pitch in the sub scanning direction in the single-side scanningmode. Meanwhile, when the double-side scanning mode takes over, thescanning pitch becomes narrower than what it was before in thesingle-side scanning mode. Due to this, during scanning on the surfaceof the photosensitive member in the double-side scanning mode, areasscanned with the light beam on the surface of the photosensitive memberexcessively overlap each other between adjacent scanning lines. Sincelatent images formed on the adjacent scanning lines excessively overlapwith each other, more than necessary toner adheres, which in turn causesa problem of an impaired image whose line is too thick or which iscolored in excessively dark shades.

Means which attains tone reproduction described above may be a linescreen which changes the line width of a line extending in apredetermined direction in accordance with a tone level for tonereproduction, a halftone screen which grows the sizes of halftone dotswhich are spaced apart in a predetermined direction in accordance with atone level for tone reproduction, etc.

However, in the event that the structure above capable of switchingbetween the double-side scanning mode and the single-side scanning modeneeds to form a latent image on the photosensitive member in thedouble-side scanning mode, image impairment could occur due to thephenomenon that the scanning pitch in the sub scanning direction doesnot remain constant. The reason will now be described in details. Whilea beam spot scans over the surface of a photosensitive member and alatent image is formed in an image forming apparatus as that describedabove, the scanning pitch in the sub scanning direction is not constantin the double-side scanning mode, and overlapping of beam spots in thesub scanning direction varies. In short, large beam spot overlaps arecreated in the sub scanning direction in areas where the scanning pitchin the sub scanning direction is narrow, whereas small beam spotoverlaps are created in the sub scanning direction in areas where thescanning pitch in the sub scanning direction is wide. Hence, when onetries forming a line which extends in a predetermined direction for tonereproduction using a line screen, the line will become thin within areaswith large beam spot overlaps created in the sub scanning direction butthick within areas with small beam spot overlaps created in the subscanning direction. This will result in an unwanted pattern of the linebecoming sometimes thin and sometimes thick due to the uneven scanningpitch in the sub scanning direction, and hence, image impairment offailing to attain favorable tone reproduction could occur. Similar imageimpairment could occur during tone reproduction using a halftone screenin which halftone dots are spaced apart in a predetermined direction.

Further, a tone reproduction characteristic significantly changesbetween when the image forming apparatus described above forms a latentimage on the photosensitive member in the double-side scanning mode andwhen it forms a latent image on the photosensitive member in thesingle-side scanning mode. The reasons will now be described in details.The first reason is as follows. The scanning pitch in the sub scanningdirection is narrower in the double-side scanning mode than in thesingle-side scanning mode. Hence, in the double-side scanning mode, beamspots scanning over the surface of a photosensitive member overlap eachother more significantly between adjacent scanning lines than in thesingle-side scanning mode. Toner could therefore adhere in greateramounts to these beam spot overlaps in the double-side scanning modethan in the single-side scanning mode, and shades of a color couldbecome darker. The following is the second reason. In the image formingapparatus described above, while the scanning pitch in the sub scanningdirection is constant in the single-side scanning mode, the scanningpitch in the sub scanning direction does not stay constant in thedouble-side scanning mode. In the event that a beam spot is to scan onthe surface of the photosensitive member to form a latent image in thedouble-side scanning mode therefore, overlapping of beam spots in thesub scanning direction varies due to the fluctuating scanning pitch inthe sub scanning direction. In other words, while the beam spot overlapsin the sub scanning direction are large in areas where the scanningpitch in the sub scanning direction is narrow, the beam spot overlaps inthe sub scanning direction are small in areas where the scanning pitchin the sub scanning direction is wide. Due to this, in the double-sidescanning mode, the fluctuating scanning pitch in the sub scanningdirection could result in a color which spans multiple shades. The tonereproduction characteristic is thus remarkably different between when alatent image is formed on the photosensitive member in the double-sidescanning mode and when a latent image is formed on the photosensitivemember in the single-side scanning mode. Such a tone reproductioncharacteristic difference could serve as a major obstacle against suchan apparatus above which performs image formation while switchingbetween the double-side scanning mode and the single-side scanning modeas required when the apparatus tries to realize excellent tonereproduction in any scanning mode. That is, for instance, even thoughexcellent tone reproduction is attained in the single-side scanningmode, after switching to the double-side scanning mode, for the reasonsabove, dark shades could be dominant or an unwanted pattern could becreated. On the contrary, even when excellent tone reproduction isattained in the double-side scanning mode, after switching to thesingle-side scanning mode, light shades could be dominant.

Further, in an image forming apparatus which uses such a light scanningapparatus, at the stage that a deflection mirror oscillates in sinemotions and makes a light beam scan on the surface of a photosensitivemember which is a surface to be scanned, if the scanning is realized viaan imaging optical system exhibiting an arc sign lens characteristic,the light beam reciprocally scans over the surface of the photosensitivemember at an equal speed in the main scanning direction. While the lightbeam reciprocally scans over the surface of the photosensitive member asdescribed above, the surface of the photosensitive member is driven inthe sub scanning direction which is approximately orthogonal to the mainscanning direction. In such an image forming apparatus therefore, thescanning pitch in the sub scanning direction is not constant, which willbe described in details later. Hence, when one wishes to form a lineimage extending in the sub scanning direction for instance, in areaswhere the scanning pitch in the sub scanning direction is wide beamspots which are created as the light beam is imaged on the surface ofthe photosensitive member fail to overlap with each other in the subscanning direction. Then image impairment that the line image becomesdiscontinuous could occur.

An advantage of some aspects of the invention is to make such an imageforming apparatus and such an image forming method, in which anoscillating deflection mirror makes a latent image forming lightirradiated in the main scanning direction upon a latent image carrierwhich is driven in the sub scanning direction and a latent image isconsequently formed, capable of switching between the single-sidescanning mode and the double-side scanning mode, and to provide atechnique for forming an excellent image while preventing adhesion ofmore than necessary toner during development of a latent image formed ona photosensitive member in the double-side scanning mode.

An advantage of some aspects of the invention is to make such an imageforming apparatus, in which an oscillating deflection mirror makes abeam spot scan in the main scanning direction on a latent image carrierwhich is driven in the sub scanning direction and a latent image isconsequently formed, capable of switching between the single-sidescanning mode and the double-side scanning mode, and to provide atechnique for realizing excellent tone reproduction whilepreventing-impairment of this image even in the double-side scanningmode.

An advantage of some aspects of the invention is to make such an imageforming apparatus, in which an oscillating deflection mirror makes abeam spot scan in the main scanning direction over a latent imagecarrier which is driven in the sub scanning direction and a latent imageis consequently formed, capable of switching between the single-sidescanning mode and the double-side scanning mode, and to provide atechnique for realizing excellent tone reproduction in any one of thesingle-side scanning mode and the double-side scanning mode.

An advantage of some aspects of the invention is to ensure, in a lightscanning apparatus in which a deflection mirror makes a light beam scanon a surface to be scanned which is driven in the sub scanningdirection, that excellent two-dimensional scanning is attained with beamspots connected to each other without any break even in an area wherethe scanning pitch is wide.

An advantage of some aspects of the invention is to provide an imageforming apparatus which is capable of forming a favorable image usingsuch a light scanning apparatus as that described above.

According to a first aspect of the invention, there is provided an imageforming apparatus, comprising: a latent image carrier whose surfaceincludes an effective image region spanning across a predetermined widthin a main scanning direction and is driven in a sub scanning directionapproximately orthogonal to the main scanning direction; a latent imageformer which has a light source and a deflection mirror oscillating, anddeflects a light beam from the light source using the deflection mirrorso as to scan the effective image region with the deflected light beam;and a scanning mode controller which switches selectively between asingle-side scanning mode and a double-side scanning mode, thesingle-side scanning mode being a mode in which the light beam isscanned only in a first direction included in the main scanningdirection, the double-side scanning mode being a mode in which the lightbeam is scanned in both the first direction and a second directionopposite to the first direction, wherein a condition to form latentimages on the latent image carrier in the single-side scanning mode isdifferent from a condition to form latent images on the latent imagecarrier in the double-side scanning mode.

According to a second aspect of the invention, there is provided acontrol method for an image forming apparatus comprising: a latent imagecarrier whose surface includes an effective image region spanning acrossa predetermined width in a main scanning direction and is driven in asub scanning direction approximately orthogonal to the main scanningdirection; and a latent image former which has a light source and adeflection mirror oscillating, and deflects a light beam from the lightsource using the deflection mirror so as to scan the effective imageregion with the deflected light beam, the method comprising of:executing a single-side scanning mode in which the light beam is scannedonly in a first direction included in the main scanning direction;executing a double-side scanning mode in which the light beam is scannedin both the first direction and a second direction opposite to the firstdirection; and switching selectively between the single-side scanningmode and the double-side scanning mode, wherein a condition to formlatent images on the latent image carrier in the single-side scanningmode is different from a condition to form latent images on the latentimage carrier in the double-side scanning mode.

According to a third aspect of the invention, there is provided a lightscanning apparatus comprising: a light source which emits a light beam;a deflector which has a deflection mirror oscillating in sine motionsabout a drive axis approximately orthogonal to a main scanningdirection, the deflection mirror reflecting the light beam emitted fromthe light source so as to scan the light beam reciprocally in a mainscanning direction; and an imaging optical system which exhibits anarc-sign theta lens characteristic, and focuses the light beam deflectedby the deflector on a surface to be scanned so as to form a beam spot onthe surface, the surface being driven in a sub scanning directionapproximately orthogonal to the main scanning direction and including aneffective scan region spanning across a predetermined width in the mainscanning direction, wherein a diameter of the beam spot in the subscanning direction is equal to or larger than the maximum scanning pitchin the sub scanning direction within the effective scan region.

According to a forth aspect of the invention, there is provided an imageforming apparatus, comprising: a latent image carrier whose surfaceincludes an effective scan region spanning across a predetermined widthin a main scanning direction and is driven in a sub scanning directionapproximately orthogonal to the main scanning direction; a light sourcewhich emits a light beam; a deflector which has a deflection mirroroscillating in sine motions about a drive axis approximately orthogonalto a main scanning direction, the deflection mirror reflecting the lightbeam emitted from the light source so as to scan the light beamreciprocally in a main scanning direction; and an imaging optical systemwhich exhibits an arc-sign theta lens characteristic, and focuses thelight beam deflected by the deflector on the surface of the latent imagecarrier so as to form a beam spot on the surface, wherein a diameter ofthe beam spot in the sub scanning direction is equal to or larger thanthe maximum scanning pitch in the sub scanning direction within theeffective scan region.

The above and further objects and novel features of the invention willmore fully appear from the following detailed description when the sameis read in connection with the accompanying drawing. It is to beexpressly understood, however, that the drawing is for purpose ofillustration only and is not intended as a definition of the limits ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an image forming apparatus according to afirst embodiment of the invention.

FIG. 2 is a block diagram showing an electrical arrangement of the imageforming apparatus of FIG. 1.

FIG. 3 is a sectional view taken on a main scan direction for showing anarrangement of the exposure unit (light scanning apparatus) provided inthe image forming apparatus of FIG. 1.

FIG. 4 is a diagram showing a scan region of the light beam in theexposure unit of FIG. 3.

FIG. 5 is a diagram showing signal processor blocks of the image formingapparatus of FIG. 1.

FIG. 6A is a diagram showing line latent image formed with light beam infirst direction.

FIG. 6B is a diagram showing line latent image formed with light beam insecond direction

FIGS. 7A, 7B and 7C are diagrams showing the relation between beam widthand scanning pitch.

FIG. 8 is a flow chart of an operation of the image forming apparatusaccording to the first embodiment.

FIG. 9 is a drawing of latent images which are formed as a result of thelatent image forming operation according to this embodiment.

FIG. 10 is a drawing showing overlapping of the light beam in thedouble-side scanning mode.

FIG. 11A is a drawing showing resultant toner images in double-sidescanning mode.

FIG. 11B is a drawing showing resultant toner images in single-sidescanning mode.

FIGS. 12A, 12B and 12C are explanatory diagrams illustrating means whichrealizes tone reproduction.

FIG. 13 is a flow chart of an operation of the image forming apparatusaccording to the second embodiment.

FIG. 14 is a drawing of latent images which are formed as a result ofthe latent image forming operation according to the second embodiment.

FIG. 15 is an explanatory diagram on the scanning pitch in thedouble-side scanning mode.

FIG. 16 is a diagram showing scanning line in a double-side scanningmode.

FIGS. 17A, 17B and 17C are drawings which show halftoning using a linescreen.

FIG. 18 is a flow chart of the latent image forming operation in thesixth embodiment.

FIG. 19A is a diagram showing single-side sub-scanning line screenangles.

FIG. 19B is a diagram showing double-side sub-scanning line screenangles.

FIG. 20 is a drawing of the scanning pitch in the double-side scanningmode.

FIG. 21 is an explanatory diagram regarding a pattern attributable tothe unevenness of the scanning pitch.

FIGS. 22A, 22B, 22C and 22D are drawings showing a relationship betweenwidth of line latent image and the angle between a line screen and thesub scanning direction.

FIGS. 23A, 23B and 23C are drawings showing halftone screen.

FIG. 24 is a flow chart of an operation of the image forming apparatusaccording to the seventh embodiment.

FIG. 25A is a drawing of the sub-scanning halftone screen angles in asingle-side scanning mode in the seventh embodiment.

FIG. 25B is a drawing of the sub-scanning halftone screen angles in adouble side scanning mode in the seventh embodiment.

FIGS. 26A, 26B, 26C and 26D are drawings which show a relationshipbetween the widths of the halftone dots and the angle of the arrangementdirections of the halftone dots with respect to the sub scanningdirection.

FIG. 27 is a block diagram of signal processing in the eighthembodiment.

FIG. 28 is an explanatory diagram of a dither method.

FIG. 29 is a flow chart of the tone control processing in thesingle-side scanning mode according to the eighth embodiment.

FIG. 30 is an explanatory diagram on the tone characteristic.

FIG. 31 is a flow chart of the tone control processing in thedouble-side scanning mode according to the eighth embodiment.

FIG. 32A is a drawing which shows the threshold value matrices in thesingle-side scanning.

FIG. 32B is a drawing which shows the threshold value matrices in thedouble-side scanning mode.

FIG. 33 is a flow chart of the latent image forming operation in theimage forming apparatus.

FIG. 34 is a drawing which shows a relationship between the spotdiameter and the scanning pitch.

FIG. 35 is a sectional view taken on a main scan direction for showingan arrangement of the exposure unit of another embodiment.

FIG. 36 is a sectional view taken on a main scan direction for showingan arrangement of the exposure unit of another embodiment.

FIGS. 37A, 37B and 37C are explanatory diagrams regarding means whichrealizes the sub-scanning line screen angles for the single-sidescanning mode according to the first example.

FIGS. 38A, 38B and 38C are explanatory diagrams regarding means whichrealizes the sub-scanning line screen angle for the double-side scanningmode according to the first example.

FIGS. 39A, 39B and 39C are explanatory diagrams regarding means whichrealizes the sub-scanning halftone screen angles for the single-sidescanning mode according to the second example.

FIGS. 40A, 40B and 40C are explanatory diagrams regarding means whichrealizes the sub-scanning halftone screen angles for the double-sidescanning mode according to the second example.

FIG. 41A shows a single-side threshold matrix which is used in the thirdexample.

FIG. 41B shows a double-side threshold matrix which is used in the thirdexample.

FIG. 41C shows halftone dots which grow in accordance with tone level inthe third example.

FIG. 42A shows halftoning using the single-side threshold matrix inthird example

FIG. 42B shows halftoning using the double-side threshold matrix inthird example

FIG. 43 is an explanatory diagram of the fourth example.

FIG. 44 is an explanatory diagram of the fifth example.

FIG. 45 is an explanatory diagram of the sixth example.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment

FIG. 1 is a diagram showing an image forming apparatus according to afirst embodiment. FIG. 2 is a block diagram showing an electricalarrangement of the image forming apparatus of FIG. 1. This image formingapparatus is a so-called tandem color printer, wherein photosensitivemembers 2Y, 2M, 2C, 2K for four colors of yellow (Y), magenta (M), cyan(C) and black (K), as latent image carriers, are juxtaposed in anapparatus body 5. The apparatus serves to form a full color image bysuperimposing toner images on the individual photosensitive members 2Y,2M, 2C, 2K, or to form a monochromatic image using only the toner imageof black (K). The image forming apparatus operates as follows. When anexternal apparatus such as a host computer applies a print command to amain controller 11 in response to a request from a user wanting to forman image, a CPU 111 of the main controller 11 sends a print command,based on which an engine controller 10 controls individual parts of anengine EG so as to form an image corresponding to the print command on asheet S such as copy sheet, transfer sheet, paper and transparent sheetfor OHP.

In the engine EG, charger units, developing units, exposure units (lightscanning apparatus) and cleaners are provided in correspondence to thefour photosensitive members 2Y, 2M, 2C, 2K, respectively. Thus, imageforming units, each of which includes the photosensitive member, thecharger unit, the developing unit, the exposure unit and the cleaner,are provided in association with the respective toner colors. The imageforming unit forms the toner image of each associated toner color. It isnoted that these image forming units (the charger units, developingunits, exposure units and cleaners) for the respective color componentsare arranged the same way. Therefore, the arrangement for the yellowcolor is described here while individual parts of the arrangements forthe other color components are represented by equivalent referencecharacters, respectively, and the description thereof is dispensed with.

The photosensitive member 2Y is rotatable in a direction of an arrow inFIG. 1 (sub-scan direction). More specifically, a drive motor MT ismechanically connected to one end of the photosensitive member 2Y. Themotor controller 105 connected with the drive motor MT electrically,controls the drive motor MT. Thus, the photosensitive member 2Y isrotatably moved. According to this embodiment, the photosensitive member2Y is driven by transmitting a drive force from the drive motor MT onlyto the one end of the photosensitive member 2Y. Furthermore, thisembodiment is designed such that a location of the drive motor MT, alocation of a horizontal synchronous sensor 60 to be describedhereinlater and a scan direction of a light beam satisfy a predeterminedrelation.

Around the photosensitive member 2Y driven in this manner, a chargerunit 3Y, a developing unit 4Y and a cleaner (not shown) are arranged ina rotational direction thereof. The charger unit 3Y comprises ascorotron charger, for example, which is applied with a charging biasfrom a charge controller 103 thereby uniformly charging an outsidesurface of the photosensitive member 2Y to a predetermined surfacepotential. An exposure unit 6Y emits a scan light beam Ly toward theoutside surface of the photosensitive member 2Y so charged by thecharger unit 3Y. Thus, an electrostatic latent image corresponding toyellow-image data included in the print command is formed on thephotosensitive member 2Y. The exposure unit 6Y is equivalent to a“latent image former” of the invention and operates according to acontrol command from an exposure controller 102Y (FIG. 4). Arrangementsand operations of the exposure unit 6 (6Y, 6M, 6C, 6K) and the exposurecontroller 102 (102Y, 102M, 102C, 102K) will be described in detailshereinlater.

The electrostatic latent image thus formed is developed with toner bymeans of the developing unit 4Y (developer). The developing unit 4Ycontains therein a yellow toner. When a developing unit controller 104applies a developing bias to a developing roller 41Y, the toner carriedon the developing roller 41Y is made to locally adhere to surfaceportions of the photosensitive member 2Y according to the surfacepotentials thereof. As a result, the electrostatic latent image on thephotosensitive member 2Y is visualized as a yellow toner image. A DCvoltage or a DC voltage superimposed with an AC voltage may be used asthe developing bias to be applied to the developing roller 41Y.Particularly in an image forming apparatus of a non-contact developmentsystem wherein the photosensitive member 2Y is spaced away from thedeveloping roller 41Y and the toner is made to jump between thesemembers for accomplishing the development with toner, the developingbias may preferably have a waveform formed by superimposing asinusoidal-wave, triangular-wave or rectangular-wave AC voltage on theAC voltage such as to effect efficient toner jumps.

The yellow toner image developed by the developing unit 4Y is primarilytransferred onto an intermediate transfer belt 71 of a transfer unit 7in a primary transfer region TRy1. The other members for the other colorcomponents than yellow are arranged absolutely the same way as those forthe yellow. A magenta toner image, a cyan toner image and a black tonerimage are formed on the respective photosensitive members 2M, 2C, 2K andare primarily transferred onto the intermediate transfer belt 71 inrespective primary transfer regions TRm1, TRc1, TRk1.

The transfer unit 7 includes: the intermediate transfer belt 71entrained about two rollers 72, 73; and a belt driver (not shown) fordriving the roller 72 into rotation thereby rotating the intermediatetransfer belt 71 in a predetermined rotational direction R2. Thetransfer unit is further provided with a secondary transfer roller 74which opposes the roller 73 with the intermediate transfer belt 71interposed therebetween and which is adapted to be moved into contactwith or away from a surface of the belt 71 by means of an unillustratedelectromagnetic clutch. In a case where a color image is transferred tothe sheet S, primary transfer timings are controlled to superimpose theindividual toner images on each other so as to form the color image onthe intermediate transfer belt 71. Then, the color image is secondarilytransferred onto the sheet S taken out from a cassette 8 and deliveredto a secondary transfer region TR2 between the intermediate transferbelt 71 and the secondary transfer roller 74. In a case where amonochromatic image is transferred to the sheet S, only a black tonerimage is formed on the photosensitive member 2K and the monochromaticimage is secondarily transferred onto the sheet S delivered to thesecondary transfer region TR2. The sheet S thus receiving thesecondarily transferred image is transported to a discharge tray on atop surface of the apparatus body via a fixing unit 9.

After the primary transfer of the toner images to the intermediatetransfer belt 71, the surface potentials of the photosensitive members2Y, 2M, 2C, 2K are reset by unillustrated static eliminators. Inaddition, the photosensitive members are removed of the toners remainingon their surfaces by means of the cleaners. Then, the photosensitivemembers are subjected to the subsequent charging by means of the chargerunits 3Y, 3M, 3C, 3K.

Disposed in the vicinity of the roller 72 are a transfer belt cleaner75, a density sensor 76 (FIG. 2) and a vertical synchronous sensor 77(FIG. 2). Of these, the cleaner 75 is adapted to be moved into contactwith or away from the roller 72 by means of an unillustratedelectromagnetic clutch. As moved to the roller 72, the cleaner 75 holdsits blade against the surface of the intermediate transfer belt 71entrained about the roller 72 thereby removing the toner remaining onthe outside surface of the intermediate transfer belt 71 after thesecondary image transfer. The density sensor 76 confronts the surface ofthe intermediate transfer belt 71 for sensing optical densities of patchimages formed as tonal patch images on the outside surface of theintermediate transfer belt 71. The vertical synchronous sensor 77 is asensor for detecting a reference position on the intermediate transferbelt 71. The sensor functions as a vertical synchronous sensor foroutputting a synchronous signal or a vertical synchronous signal Vsyncin association with a drivable rotation of the intermediate transferbelt 71 in the sub-scan direction. In this apparatus, the operations ofthe individual parts of the apparatus are controlled based on thevertical synchronous signal Vsync for the purposes of synchronizing theoperation timings of the individual parts and precisely superimposingthe toner images of the respective colors on each other.

In FIG. 2, a reference numeral 113 represents an image memory providedin the main controller 11 for storing image data supplied from theexternal apparatus, such as the host computer, via an interface 112. Areference numeral 106 represents a ROM for storing operation programsexecuted by the CPU 101, control data used for controlling the engineEG, and the like. A reference numeral 107 represents a RAM fortemporarily storing the operation results given by the CPU 101, andother data. Further, denoted at 108 is an FRAM (ferroelectric memory)which saves information related to the statuses of use of the respectiveportions of the engine.

FIG. 3 is a sectional view taken on a main scan direction for showing anarrangement of the exposure unit (light scanning apparatus) provided inthe image forming apparatus of FIG. 1. FIG. 4 is a diagram showing ascan region of the light beam in the exposure unit of FIG. 3. FIG. 5 isa diagram showing signal processor blocks of the image forming apparatusof FIG. 1. Referring to these figures, the arrangements and operationsof the exposure unit 6 and the exposure controller 102 are specificallydescribed as below. The exposure unit 6 and the exposure controller 102for the respective color components are arranged the same ways.Therefore, the arrangement for the yellow color is described here whilethe individual parts of the arrangements for the other color componentsare represented by equivalent reference characters, respectively, andthe description thereof is dispensed with.

The exposure unit 6Y (6M, 6C, 6K) includes an exposure casing 61. Theexposure casing 61 has a single exposure light source 62Y fixed theretoso as to be capable of emitting the light beam from the laser lightsource 62Y. The laser light source 62Y is electrically connected with alight source driver 1021 of the exposure controller 102Y shown in FIG.5. The light source driver 1021 operates as follows to provide ON/OFFcontrol of the laser light source 62Y according to an image signal, sothat the laser light source 62Y emits the light beam modulated incorrespondence to the image data. Referring to FIG. 5, description ismade as below.

In this image forming apparatus, upon receipt of an image signal from anexternal apparatus such as a host computer 100, the main controller 11performs predetermined signal processing of the image signal. The maincontroller 11 comprises functional blocks such as a color converter 114,an image processor 115, two types of line buffers 116A and 116B, ascanning mode switcher 116C, a pulse modulator 117, a tone correctiontable 118 and a correction table calculator 119.

In addition to the CPU 101, the ROM 106, and the RAM 107 shown in FIG.2, the engine controller 10 further includes a tone characteristicdetector 123 for detecting a tone characteristic of the engine EG basedon a detection result given by the density sensor 76, the tonecharacteristic representing a gamma characteristic of the engine EG. Inthe main controller 11 and the engine controller 10, these functionblocks may be implemented in hardware or otherwise, in software executedby the CPU 111, 101.

In the main controller 11 supplied with the image signal from the hostcomputer 100, the color converter 114 converts RGB tone data intocorresponding CMYK tone data, the RGB tone data representing therespective tone levels of RGB components of each pixel in an imagecorresponding to the image signal, the CMYK tone data representing therespective tone levels of CMYK components corresponding to the RGBcomponents. In the color converter 114, the input RGB tone data comprise8 bits per color component for each pixel (or representing 256 tonelevels), for example, whereas the output CMYK tone data similarlycomprise 8 bits per color component for each pixel (or representing 256tone levels). The CMYK tone data outputted from the color converter 114are inputted to the image processor 115.

The image processor 115 performs the following processes on each of thecolor components. That is, the image processor performs tone correctionand a half-toning process on the per-pixel tone data inputted from thecolor converter 114. Specifically, the image processor 115 refers to thetone correction table 118 previously stored in a non-volatile memory,and converts the per-pixel tone data inputted from the color converter114 into corrected tone data according to the tone correction table 118,the corrected tone data representing corrected tone levels. An object ofthe tone correction is to compensate for the variations of the gammacharacteristic of the engine EG constructed as described above, therebyto maintain the overall gamma characteristic of the image formingapparatus in an idealistic state at all times. In the image formingapparatuses of this type, the gamma characteristic varies from oneapparatus to another. In addition, the apparatus per se encounters thevaried gamma characteristic according to use conditions. In order toeliminate influences of the varied gamma characteristic upon the imagequality, a tone control process is performed in predetermined timingsfor updating the contents of the aforementioned tone correction table118 based on measured image densities.

The tone control process is performed as follows. The tonal patch imagesfor tone correction, which are previously defined for measurement of thegamma characteristic, are formed on the intermediate transfer belt 71 bythe engine EG on a per-toner-color basis. The respective densities ofthe tonal patch images are sensed by the density sensor 76. Based onsignals from the density sensor 76, the tone characteristic detector 123generates a tone characteristic (the gamma characteristic of the engineEG) wherein the respective tone levels of the tonal patch images are incorrespondence to the respective image densities thus detected. Theresultant tone characteristic is outputted to the correction tablecalculator 119 of the main controller 11. The correction tablecalculator 119, in turn, operates tone correction table data forobtaining the idealistic tone characteristic, as compensating for themeasured tone characteristic of the engine EG based on the tonecharacteristic supplied from the tone characteristic detector 123. Thecorrection table calculator 119 updates the contents of the tonecorrection table 118 according to the operation results. In this manner,the tone correction table 118 is redefined. By making such updates, theimage forming apparatus is adapted to form images of a consistentquality irrespective of the variations of the gamma characteristic ofthe apparatus or time-related changes thereof.

The image processor 115 halftones the corrected tone data thuscorrected, during which one halftone dot is formed using multiplepixels, and the size of the halftone dot is enlarged by a dither method,an error diffusion method, a screen method or the like and a tone isrealized, and the image processor 115 feeds halftoned tone data whichcontain eight bits per halftone dot per color to the two types of linebuffers 116A and 116B. The content of the halftoning is differentdepending upon an image to form. In short, in accordance with acriterion such as whether the image is a monochrome image or a colorimage and whether the image is a line image or a photo image, an optimalprocessing content to the image is selected and executed.

Although the line buffers 116A and 116B are common to each other in thatthey store halftoned tone data (image information) constituting the1-line image data output from the image processor 115, they read thetone data in different orders. That is, while the forward-direction linebuffer 116A outputs the halftoned tone data constituting the 1-lineimage data in the forward direction from the beginning, thereverse-direction line buffer 116B outputs in the reverse direction fromthe end.

The scanning mode switcher 116C receives thus output halftoned tonedata, and based on a scanning mode switching signal, outputs atappropriate timing to the pulse modulator 117 only the halftoned tonedata output from one of the line buffers. The principal reason ofdisposing the two types of line buffers 116A and 116B is to deal withdifferent scanning modes for the light beam in accordance with theprinting mode as described later. The scanning mode switcher 116Cfurther ensures that the pulse modulator 117 receives the tone data atsuch timing and in such an order corresponding to each color component.In this embodiment, the line buffers 116A and 116B and the scanning modeswitcher 116C thus correspond to the “scanning mode controller” of theinvention.

The halftoned tone data inputted to the pulse modulator 117 arerepresented by multivalued signals which indicate respective sizes ofdots of color toners to be made to adhere to each pixel and an array ofthe toner dots. Receiving such data, the pulse modulator 117 uses thehalf-toned tone data to generate a video signal for pulse widthmodulation of an exposure laser pulse used by the engine EG to form animage of each color. The pulse modulator 117 outputs the video signal tothe engine controller 10 via an unillustrated video interface. Alight-source driver 1021 of the exposure controller 102Y, receiving thevideo signal, provides ON/OFF control of the laser light source 62Y ofthe exposure unit 6. The same operations are performed on the othercolor components.

Returning to FIG. 3 and FIG. 4, further explanation is made as follows.Provided in the exposure casing 61 are a collimator lens 631, acylindrical lens 632, a deflector 65 and a scanning lens 66 such as toscan the light beam from the laser light source 62Y on the surface (notshown) of the photosensitive member 2Y. Specifically, the light beamfrom the laser light source 62Y is shaped into a collimated beam of asuitable size by means of the collimator lens 631 and then is madeincident on the cylindrical lens 632 powered only in a sub-scandirection Y. By adjusting the cylindrical lens 632, the collimated beamis focused onto place near a deflective mirror surface 651 of thedeflector 65 with respect to the sub-scan direction Y. According to theembodiment, a combination of the collimator lens 631 and the cylindricallens 632 functions as a beam shaping system 63 for shaping the lightbeam from the laser light source 62Y

The deflector 65 is formed, using a micromachining technique whichutilizes a semiconductor fabrication technique for integrally formingmicro machines on a semiconductor substrate. The deflector comprises adeflection mirror adapted for resonant oscillations. Specifically, thedeflector 65 is capable of deflecting the light beam in a main scandirection X by means of the deflective mirror surface 651 in resonantoscillations. More specifically, the deflective mirror surface 651 isoscillatbly mounted about an oscillatory axis (torsion spring) extendingsubstantially perpendicular to the main scan direction X. The deflectivemirror sinusoidally oscillates about the oscillatory axis according toan external force applied from an operating section (not shown). Theoperating section applies an electrostatic, electromagnetic ormechanical external force to the deflective mirror surface 651 based ona mirror drive signal from a mirror driver (not shown) of the exposurecontroller 102, thereby bringing the deflective mirror surface 651 intooscillations at a frequency of the mirror drive signal. The operatingsection may adopt any of the drive methods based on electrostaticattraction, electromagnetic force and mechanical force. These drivemethods are known in the art and hence, the description thereof isdispensed with.

The light beam deflected by the deflective mirror surface 651 of thedeflector 65 is directed toward the scanning lens 66 at a maximumoscillation angle θmax, as shown in FIG. 4. In this embodiment, thescanning lens 66 is designed to have a substantially constant F-valuewith respect to the overall effective image region IR in other words,effective scan region ESR on the photosensitive member 2. Therefore, thelight beam deflected toward the scanning lens 66 passes therethrough tobe focused on the effective image region IR on the photosensitive membersurface 22 in a spot of a substantially constant diameter. Thus, thelight beam is scanned in parallel to the main scan direction X so as toform a linear latent image on the effective image region IR on thephotosensitive member 2, the linear latent image extending in the mainscan direction X. As shown in FIG. 4, the embodiment defines a scanregion SR, which can be scanned by the deflector 65, (“second scanregion” of the invention) SR2 to be broader than a scan region (“firstscan region” of the invention) SR1 where the light beam is scanned onthe effective image region IR. Furthermore, the first scan region SR1 ispositioned substantially centrally of the second scan region SR2, so asto be substantially symmetrical with respect to a light axis. A symbolθir in the figure represents the oscillation angle of the deflectivemirror surface 651, which corresponds to an end of the effective imageregion IR. A symbol θs represents the oscillation angle of thedeflective mirror surface 651, which corresponds to a horizontalsynchronous sensor to be described as below.

Further, the image forming apparatus, having this structure, is capableof switching between the single-side scanning mode in which the lightbeam scans only in a first direction included in the main scanningdirection X and the double-side scanning mode in which the light beamscans in two directions, one being the first direction and the otherbeing a second direction which is opposite to the first direction. Whenone wishes to form a line latent image on the latent image carrier asshown in FIGS. 6A and 6B for example, the light beam scanning in thefirst direction included in the main scanning direction X forms linelatent images LI(+X) within an effective image region and the light beamscanning in the second direction which is opposite to the firstdirection forms line latent images LI(−X). Shown in FIGS. 6A and 6B areline latent images which the image forming apparatus shown in FIG. 1forms. In the double-side scanning mode therefore which makes the lightbeam for latent image formation scan in the first direction and thesecond direction, the line latent images LI(+X) and LI(−X) are formedalternately in the sub scanning direction. On the contrary, in thesingle-side scanning mode which makes the light beam scan in either oneof the first direction and the second direction, either line latentimages LI(+X) or line latent images LI(−X) are formed in the subscanning direction.

In addition, the image forming apparatus having this structure iscapable of making the light beam scan in the main scanning directionback and forth. That is, the light bean can scan in both the direction(+X) and the direction (−X). The tone data constituting the 1-line imagedata are temporarily stored in a storage part (the line buffers 116A and116B) as described above, and the scanning mode switcher 116C providesthe pulse modulator 117 with the tone data at appropriate timing and ina proper order. For instance, when the direction is switched to thedirection (+X), as shown in FIG. 6A, the tone data are read out from theline buffer 116A in the order of DT1, DT2, . . . DTn and beam spots areirradiated upon the photosensitive member 2 in the first direction (+X)based on each piece of the tone data, whereby line latent images LI(+X)are formed. On the contrary, when the direction is switched to thedirection (−X), as shown in FIG. 6B, the tone data are read out from theline buffer 116B in the order of DTn, DT(n−1), . . . DT1 and beam spotsare irradiated upon the photosensitive member 2 in the second direction(−X) based on each piece of the tone data, whereby line latent imagesLI(−X) are formed. The light beam for latent image formation is thuschanged for different printing modes or different lines. Describing inmore specific details, in this embodiment, the RAM 107 temporarilystores information related to a resolution (resolution information)contained in a print command. In the event that printing at a highresolution is instructed, latent images are formed as the so-calleddouble-side scanning mode is executed which alternately repeats anoperation of making a light beam SL1 scan over the effective imageregion IR in the direction (+X) and forming latent images in theeffective image region IR and an operation of making a light beam SL2scan over the effective image region IR in the direction (−X) andforming latent images in the effective image region IR. In contrast,when printing at a low resolution is instructed, latent images areformed as the so-called single-side scanning mode is executed whichrepeats only the light beam SL1. This embodiment thus demands that thescanning mode for the light beam is switched between high-resolutionprinting and low-resolution printing in accordance with the resolutioninformation. This will be described in more detail later.

As described above, according to this embodiment, the light beam havinga constant spot diameter within the effective image region IR on thesurface of the photosensitive member 2 can scan while the mode isswitched between the single-side scanning mode and the double-sidescanning mode. Further, in this embodiment, the beam width Wb of thelight beam in the sub scanning direction is equal to or wider than ascanning pitch PT which is the pitch in the single-side scanning mode.The reason of this will now be described with reference to FIGS. 7A, 7Band 7C. However, In FIGS. 7A, 7B, and 7C, the dotted-and-dashed linesare virtual lines which are indicative of the track of the scanninglines in the single-side scanning mode, while the solid lines expressthe light beam. When the beam width Wb in the sub scanning direction issmaller in the single-side scanning mode than the scanning pitch PT forthe single-side scanning mode, latent images formed as a result ofirradiation with the light beam do not connect with each other betweenadjacent scanning lines and a continuous line is not formed as shown inFIG. 7A. The line formed in the sub scanning direction is thusdiscontinued. To avoid cutting of the line formed in the sub scanningdirection, as shown in FIG. 7B or 7C, the beam width Wb on the surfaceof the photosensitive member in the sub scanning direction must be equalto or wider than the scanning pitch PT in the sub scanning direction onthe surface of the photosensitive member for the single-side scanningmode. The beam width Wb in the sub scanning direction herein referred tois a width in the sub scanning direction of a region in a lightintensity distribution (hereinafter referred to as the “beam profile”)at a position of a surface of the photosensitive member, the regionhaving an intensity of not less than 1/e² (where the symbol “e” denotesthe bottom of natural logarithm) of a peak value in the light intensitydistribution. It is possible to measure the beam width Wb of the lightbeam in the sub scanning direction by the following methods for example.As a measuring instrument, BeamScan Model 2180 manufactured by Photon,inc. may be used. The beam width Wb in the sub scanning direction isidentified through measurement of the beam profile at such locationswhich are assumed to be the surface of the photosensitive member duringcontinuous irradiation of laser of 1 mW in laser power. In general, thespot diameter of the light beam is proportional to the product of thewavelength of the light beam and the F-value within the effective imageregion IR on the surface of the photosensitive member. Hence, adjustmentof the wavelength of the light beam or the F-value makes it possible toadjust the beam width Wb in the sub scanning direction measured byeither method above to or beyond the scanning pitch PT for thesingle-side scanning mode.

Further, in this embodiment, the location of the drive motor MT relativeto this scanning direction is set in advance so as to satisfy thefollowing relationship. That is, the drive motor MT is disposed on thedownstream side in the scanning direction (+X). In addition, on theupstream side in the scanning direction (+X), a return mirror 69 guidesthe scanning light beam back to the optical sensor 60 at the end of thescanning route of the scanning light beam as shown in FIG. 3. The returnmirror 69 is disposed in an end portion of the scan region SR which islocated on the upstream side in the scanning direction (+X), and guidesback to the optical sensor 60 the scanning light beam which movesoutside the effective image region IR within the scan region SR on theupstream side in the scanning direction (+X). When the optical sensor 60receives the scanning light beam and the scanning light beam movespassed the sensor location (oscillation angle θs), the optical sensor 60outputs a signal. In this embodiment, the horizontal synchronizationsensor 60 thus functions as a horizontal synchronization read sensorwhich is for obtaining a synchronizing signal in accordance with whichthe light beam scans over the effective image region IR in the mainscanning direction X, namely, a horizontal synchronizing signal Hsyncbased on which a latent image forming operation is controlled. Thelatent image forming operation according to this embodiment will now bedescribed.

FIG. 8 is a flow chart of an operation of the image forming apparatusaccording to the first embodiment. FIG. 9 is a drawing of latent imageswhich are formed as a result of the latent image forming operationaccording to this embodiment. In FIG. 9, the dotted-and-dashed lines arevirtual lines which are indicative of the track of the scanning lines,while the thick arrows are indicative of the light beam.

Upon receipt of a print command from an external apparatus such as thehost computer 100, latent images are formed on the respectivephotosensitive members and a color image is formed from these latentimages in accordance with the flow chart in FIG. 8. In other words, atStep S10, resolution information contained in the print command isacquired (information acquiring step). Based on the resolutioninformation, whether the print command calls for printing at a highresolution or a low resolution is determined (Step S11).

When it is determined YES at Step S11, that is, when it is determinedprinting at a low resolution is demanded, Step S16 to Step S19 areexecuted. Through these Steps images are formed at a low resolution andtransferred onto a sheet S and printing is terminated. First, at StepS16, the apparatus is set to the single-side scanning mode (scanningmode setting step). Next, the level of a light-source drive signal fedto the light source 62 from the light-source driver 1021 disposed to theexposure controller 102 is set to a single-side scanning drive level(Step S17). In consequence, at Step S19 which will be described later,the light beam scanning over the photosensitive member 2 will have theamount corresponding to the single-side scanning drive level. Further,the scanning mode switching signal which corresponds to the scanningmode determined in the manner above is supplied to the scanning modeswitcher 116C of the main controller 11 (Step S18). Receiving theinstruction, the scanning mode switcher 116C fixes the timing at whichand the order in which tone data should be read from the correspondingline buffer, and forms latent images line by line. In short, the tonedata are read from the forward-direction line buffer 116A at propertiming in the forward direction (i.e., the tone data in the order ofDT1, DT2, . . . DTn), and only the light beam SL1, while being modulatedbased on the respective pieces of tone data, scans over thephotosensitive member 2 in the first direction as shown in the bottomsection of FIG. 9, whereby latent images are formed (Step S19). Theso-called single-side scanning mode is executed in this fashion, andlatent images are formed at a low resolution. Thus formed latent imagesare then developed with toner, thereby forming toner images in the fourcolors. The toner images are superimposed one atop the other on theintermediate transfer belt 71, thereby forming a color image. The colorimage is thereafter transferred onto a sheet S, and printing at a lowresolution completes.

When it is determined NO at Step S11, that is, when it is determinedprinting at a high resolution is demanded, Step S12 to Step S15 areexecuted. Through these Steps which images are formed at a highresolution and transferred onto a sheet S and printing is terminated.First, at Step S12, the apparatus is set to the double-side scanningmode (scanning mode setting step). Next, the level of the light-sourcedrive signal fed to the light source 62 from the light-source driver1021 disposed to the exposure controller 102 is set to a double-sidescanning drive level which is lower than the single-side scanning drivelevel (Step S13). In consequence, at Step S15 which will be describedlater, the light beam scanning over the photosensitive member 2 willhave the amount smaller than the amount it has in the single-sidescanning mode. Further, the scanning mode switching signal whichcorresponds to the scanning mode determined in the manner above issupplied to the scanning mode switcher 116C of the main controller 11(Step S14). Receiving the instruction, the scanning mode switcher 116Cswitches the timing at which and the order in which halftoned datashould be read from the corresponding line buffer, alternately everyline. Therefore, high-resolution latent images are formed in thefollowing manner. That is, as shown in the top section of FIG. 9, anoperation of making the light beam SL1 scan over the effective imageregion IR in the direction (+X) and accordingly forming latent imageswithin the effective image region IR and an operation of making thelight beam SL2 scan over the effective image region IR in the direction(−X) and accordingly forming latent images within the effective imageregion IR are repeated alternately (Step S15). The so-called double-sidescanning mode is executed in this fashion, and latent images are formedat a high resolution. Thus formed latent images are then developed withtoner, thereby forming toner images in the four colors. The toner imagesare superimposed one atop the other on the intermediate transfer belt71, thereby forming a color image. The color image is thereaftertransferred onto a sheet S, and printing at a high resolution completes.

In the first embodiment, the double-side scanning drive level is lowerthan the single-side scanning drive level as described above. It istherefore possible to prevent adhesion of more than necessary toner inthe double-side scanning mode, and to form an excellent image. Thereason of this will now be described with reference to FIGS. 10 and 11.FIG. 10 is a drawing which shows overlapping of the light beam in thedouble-side scanning mode. FIGS. 11A and 11B are drawings which showsresultant toner images. In FIG. 10, the dotted-and-dashed line is avirtual line which is indicative of the track of the scanning lines,while the ellipses encircled by the solid lines are indicative of thelight beam. As described above, in the first embodiment, the beam widthof the light beam in the sub scanning direction is equal to or widerthan the scanning pitch for the single-side scanning mode. Since thescanning pitch reduces at the time of switching from the single-sidescanning mode to the double-side scanning mode, in a region TR shown inFIG. 10, sections on the surface of the photosensitive member scannedwith the light beam excessively overlap with each other between adjacentscanning lines. Latent images formed on these adjacent scanning linescould excessively overlap with each other due to this in the double-sidescanning mode and toner therefore adheres more than necessary, whichcauses image impairment such as too thick lines and too dark colorshades.

On the contrary, according to the first embodiment, since thedouble-side scanning drive level is lower than the single-side scanningdrive level, the amount of the light beam in the double-side scanningdrive level is smaller than that in the single-side scanning drivelevel. A latent image formed by such a small amount of the light beamneeds less toner for development. This is because for development withtoner of a latent image formed on the surface of a photosensitivemember, adhesion of toner is generally in accordance with a potentialdifference between an electric potential at a portion which bears thelatent image and a developing bias potential. In other words, the largerthis potential difference is, the more amount of toner adheres. Further,the smaller the light amount is for creation of the latent imageportion, the smaller the potential difference between the developingbias potential and the electric potential at the latent image portionis. Hence, as shown in FIGS. 11A and 11B, in the double-side scanningmode, a latent image formed with a smaller amount of light than in thesingle-side scanning mode receives less toner than in the single-sidescanning mode. It is therefore possible to obviate image impairmentattributable to excessive adhesion of toner in the double-side scanningmode and to form an excellent image. In FIGS. 11A and 11B, thedotted-and-dashed lines are virtual lines which are indicative of thetrack of the scanning lines.

Further, the first embodiment requires selectively switching between thedouble-side scanning mode and the single-side scanning mode based onresolution information to thereby switch a resolution during printing.Switching of the scanning mode for the light beam alone, withoutchanging oscillation of the deflection mirror surface 651, realizesselective execution of either high-resolution printing or low-resolutionprinting. Hence, it is possible to quickly change the resolution.

Second Embodiment

By the way, a line image such as a letter is printed favorably only whenthe line is formed continuously without any break, and therefore, doesnot demand a very high resolution. In the meantime, when one wishes toprint a photograph or the like beautifully, tone reproduction isrequired as described in details later, and to this end, an enhancedresolution is preferred. It is therefore preferable that low-resolutionprinting is carried out to print a line image such as a letter throughexecution of the single-side scanning mode, but for printing of aphotograph or the like which demands tone reproduction, the double-sidescanning mode is executed and a resolution is improved.

As described above, since tone reproduction is required for beautifulprinting of a photograph, etc., a resolution needs be increased. Thereason of this will now be described with reference to FIGS. 12A, 12Band 12C. FIGS. 12A, 12B and 12C are explanatory diagrams illustratingmeans which realizes tone reproduction. The means which realizes tonereproduction may be creation of one halftone dot using multiple pixels,size growth of the halftone dot and realization of a tone. For example,in FIG. 12A, 16 pixels in total, four pixels wide and four pixels high,form one halftone dot. From (1) toward (16), more pixels are exposed,with which the size of this halftone dot corresponding to the exposedpart grows in accordance with a predetermined rule, thereby realizingtone reproduction over 16 shades of gray. The tone level expressesprogressively darker shades from (1) toward (16). Further, as shown inFIG. 12B, (1) through (4), when only one part within one pixel isexposed, multiple tone levels are achieved within one pixel. Hence, inthe event that one pixel spans 16 tone levels and 16 pixels constituteone halftone dot, one halftone dot can have 16 tone levels×16 tonelevels=256 shades of gray. However, reproduction of many tone levelsrequiring use of many pixels gives rise to a problem that adjacenthalftone dots become more sparsely spaced and a photograph looks coarse.In light of this, for printing of a photograph, the double-side scanningmode is run as shown in FIG. 12C, thereby doubling a resolution in thesub scanning direction and halving the distances between neighboringhalftone dots from their distances in the single-side scanning mode.This attains even finer printing of a photograph. According to thesecond embodiment therefore, whether tone reproduction is necessary isdetermined, and when tone reproduction is needed, the double-sidescanning mode is run and printing at a high resolution is performed,whereas when tone reproduction is unnecessary, low-resolution printingis carried out in the single-side scanning mode. An operation of theapparatus for switching the scanning mode in accordance with whethertone reproduction is necessary will now be described in details withreference to FIGS. 13 and 14. The basic structure of the apparatusaccording to the second embodiment is the same as that of the apparatusaccording to the first embodiment, and therefore, the same structurewill be denoted at the same or corresponding reference symbols but willnot be described in redundancy.

FIG. 13 is a flow chart of an operation of the image forming apparatusaccording to the second embodiment. FIG. 14 is a drawing of latentimages which are formed as a result of the latent image formingoperation according to the second embodiment. In FIG. 14, thedotted-and-dashed lines are virtual lines which are indicative of thetrack of the scanning lines, while the thick arrows are indicative ofthe light beam. In the second embodiment, upon receipt of a printcommand from an external apparatus such as the host computer 100, latentimages are formed on the respective photosensitive members and a colorimage is formed from these latent images in accordance with the flowchart in FIG. 13. In other words, at Step S20, resolution informationcontained in the print command is acquired (information acquiring step).Whether tone reproduction is necessary is then determined (Step S21).

When it is determined NO at Step S21, that is, when it is determinedthat tone reproduction is unnecessary, Step S26 to Step S29 areexecuted. Through these Steps which images are formed and transferredonto a sheet S and printing is terminated. First, at Step S26, theapparatus is set to the single-side scanning mode (scanning mode settingstep). Next, the level of the light-source drive signal fed to the lightsource 62 from the light-source driver 1021 disposed to the exposurecontroller 102 is set to the single-side scanning drive level (StepS27). In consequence, at Step S29 which will be described later, thelight beam scanning over the photosensitive member 2 will have theamount corresponding to the single-side scanning drive level. Further,the scanning mode switching signal which corresponds to the scanningmode determined in the manner above is supplied to the scanning modeswitcher 116C of the main controller 11 (Step S28). Receiving theinstruction, the scanning mode switcher 116C fixes the timing at whichand the order in which tone data should be read from the correspondingline buffer, and forms latent images line by line. In short, the tonedata are read from the forward-direction line buffer 116A at propertiming in the forward direction (i.e., the tone data in the order ofDT1, DT2, . . . DTn), and only the light beam SL1, while being modulatedbased on the respective pieces of tone data, scans over thephotosensitive member 2 in the first direction as shown in the bottomsection of FIG. 14, whereby latent images are formed (Step S29). Theso-called single-side scanning mode is executed in this fashion, andlatent images are formed. Thus formed latent images are then developedwith toner, thereby forming toner images in the four colors. The tonerimages are superimposed one atop the other on the intermediate transferbelt 71, thereby forming a color image. The color image is thereaftertransferred onto a sheet S, and printing of letters completes.

When it is determined YES at Step S21, that is, when it is determinedthat tone reproduction is necessary, Step S22 to Step S25 are executed.Through Steps which images are formed at a high resolution andtransferred onto a sheet S and printing is terminated. First, at StepS22, the apparatus is set to the double-side scanning mode (scanningmode setting step). Next, the level of the light-source drive signal fedto the light source 62 from the light-source driver 1021 disposed to theexposure controller 102 is set to the double-side scanning drive levelwhich is lower than the single-side scanning drive level (Step S23). Inconsequence, at Step S25 which will be described later, the light beamscanning over the photosensitive member 2 will have the amount smallerthan the light amount it has in the single-side scanning mode. Further,the scanning mode switching signal which corresponds to the scanningmode determined in the manner above is supplied to the scanning modeswitcher 116C of the main controller 11 (Step S24). Receiving theinstruction, the scanning mode switcher 116C switches the timing atwhich and the order in which halftoned data should be read from thecorresponding line buffer, alternately every line. Therefore, latentimages are formed in the following manner. That is, as shown in the topsection of FIG. 14, an operation of making the light beam SL1 scan asthe light beam over the effective image region IR in the direction (+X)and accordingly forming latent images within the effective image regionIR and an operation of making the light beam SL2 scan over the effectiveimage region IR in the direction (−X) and accordingly forming latentimages within the effective image region IR are repeated alternately(Step S25). The so-called double-side scanning mode is executed in thisfashion, and latent images are formed. Thus formed latent images arethen developed with toner, thereby forming toner images in the fourcolors. The toner images are superimposed one atop the other on theintermediate transfer belt 71, thereby forming a color image. The colorimage is thereafter transferred onto a sheet S, and photo printingcompletes.

As described above, in the second embodiment, since the double-sidescanning mode is run for photo printing which requires tonereproduction, a resolution in the sub scanning direction doubles thatduring execution of the single-side scanning mode, which realizes finephoto printing. In addition, the double-side scanning drive level whichis lower than the single-side scanning drive level. This preventsadhesion of more than necessary toner in the double-side scanning mode,and attains photo printing with a favorable image.

Third Embodiment

FIG. 15 is an explanatory diagram on the scanning pitch in thedouble-side scanning mode. In FIG. 15, the dotted-and-dashed line is avirtual line which is indicative of the track of the scanning lines. InFIG. 15, as denoted at the reference symbols PT1 through PT3, thescanning pitch in the sub scanning direction is not constant in thedouble-side scanning mode. Hence, if the amount of the light beam is toolittle in the double-side scanning mode, the amount of adhering tonerreduces too much, which in turn causes a problem of image impairmentthat in a section with a wide scanning pitch, a line formed in the subscanning direction gets disconnected. Noting this, the third embodimentrequires that the double-side scanning drive level set in thelight-source driver 1021 in the double-side scanning mode is determinedin accordance with the maximum value of the scanning pitch in the subscanning direction for the double-side scanning mode. That is, thedouble-side scanning drive level is determined so that the width of atoner image in the sub scanning direction formed on the surface of thephotosensitive member 2 will be the same or beyond the maximum value ofthe scanning pitch in the sub direction in the double-side scanningmode. This structure ensures that even in a section where the scanningpitch is the maximum, the line formed in the sub scanning direction isnot cut as shown in FIG. 15 and that an excellent image is formed. Thebasic structure of the apparatus according to the third embodiment isthe same as that of the apparatus according to the first embodiment, andtherefore, the same structure will be denoted at the same orcorresponding reference symbols but will not be described in redundancy.

Fourth Embodiment

By the way, as shown in FIGS. 7B and 7C, according to the embodimentsdescribed above, the beam width of the light beam in the sub scanningdirection is equal to or wider than the scanning pitch in thesingle-side scanning mode. Because of this, the light beams overlap witheach other between adjacent scanning lines in the single-side scanningmode as shown in FIG. 7C. The influence of the degree of the overlappingof the light beams over an image which is formed in the double-sidescanning mode will now be considered. When the overlapping of the lightbeams is relatively large, even in the double-side scanning mode,sections scanned with the light beam excessively overlap with each otherbetween adjacent scanning lines. On the contrary, when the overlappingof the light beams is relatively small, in the double-side scanning modeas well, the degree of overlapping of sections scanned with the lightbeams overlap with each other between adjacent scanning lines isrelatively small. It then follows that independently of the degree ofthe overlapping of the light beams, if the amount of the light beam isconstant in the double-side scanning mode, the following imageimpairment could occur. That is, for example, significant overlappingcould lead to image impairment due to adhesion of unnecessary toner tooverlaps of the light beams in the double-side scanning mode. On thecontrary, when the degree of overlapping is small, image impairment of adisconnected line which is formed in the sub scanning direction couldoccur in the double-side scanning mode.

Noting this, the fourth embodiment requires that the double-sidescanning drive level which is set in the light-source driver 1021 in thedouble-side scanning mode is determined in accordance with the ratio ofthe beam width of the light beam in the sub scanning direction to thescanning pitch in the sub scanning direction for the single-sidescanning mode. That is, when set in accordance with this ratio, thedouble-side scanning drive level is suitable to the degree of theoverlapping of the light beams described above. Describing in morespecific details, when the degree of overlapping of the light beams islarge, the double-side scanning drive level is set low such that theamount of the light beam will be relatively small, whereas when thedegree of the overlapping of the light beams is small, the double-sidescanning drive level is set high such that the amount of the light beamwill be relatively large. This structure prevents the impairmentaddressed above. In other words, when the degree of the overlapping isrelatively significant, the amount of the light beam is suppressed,which prevents excessive toner adhesion and allows forming an excellentimage. On the contrary, when the degree of the overlapping is relativelysmall, the light beam is in a certain proper amount, and therefore, itis possible to avoid image impairment that the amount of the light beamis excessively suppressed, the amount of adhering toner decreases toomuch and a line formed in the sub scanning direction in the double-sidescanning mode gets disconnected. Hence, it is possible to form anexcellent image. The basic structure of the apparatus according to thefourth embodiment is the same as that of the apparatus according to thefirst embodiment, and therefore, the same structure will be denoted atthe same or corresponding reference symbols but will not be described inredundancy.

Fifth Embodiment

A consideration will now be given on a situation that the proportion ofthe effective image region to the scan region (scanning efficiency) issmall and the minimum scanning pitch can be viewed approximately thesame as the maximum scanning pitch in the double-side scanning mode asshown in FIG. 16. In FIG. 16, the dotted-and-dashed line is a virtualline which is indicative of the track of the scanning lines. Under theillustrated circumstance, the scanning pitch in the double-side scanningmode is considered roughly half that in the single-side scanning mode.Noting this, the fifth embodiment requires that the double-side scanningdrive level set in the light-source driver 1021 in the double-sidescanning mode is determined so that the amount of the light beam in thedouble-side scanning mode will be smaller than but at least half that inthe single-side scanning mode or more. By means of the structure whichsets the amount of the light beam in the double-side scanning mode tohalf that in the single-side scanning mode or a greater amount, thewidth in the sub scanning direction of a toner image formed in thedouble-side scanning mode is approximately half the width in the subscanning direction of a toner image formed in the single-side scanningmode or wider. This prevents image impairment that an excessivelysuppressed amount of the light beam in the double-side scanning modereduces the amount of adhering toner too much and a line formed in thesub scanning direction in the double-side scanning mode getsdisconnected, and instead, permits forming a favorable image. The basicstructure of the apparatus according to the fifth embodiment is the sameas that of the apparatus according to the first embodiment, andtherefore, the same structure will be denoted at the same orcorresponding reference symbols but will not be described in redundancy.

Sixth Embodiment

The sixth embodiment is similar to the preceding embodiments describedabove in that the image processor 115 halftones corrected tone data torealize a tone and the resulting halftoned tone data are fed to the twotypes of line buffers 116A and 116B, except for the followingdifference. That is, halftoning according to the sixth embodiment uses aline screen. As for the sixth embodiment, the difference from theearlier embodiments described above will be mainly described. The commonportions will be denoted at corresponding reference symbols but will notbe described. A line screen changes the line widths of plural linesextending in a predetermined direction in accordance with tones for tonereproduction. FIGS. 17A, 17B and 17C are drawings which show halftoningusing a line screen. During halftoning using such a line screen, asshown in FIGS. 17A through 17C, as the lines are thickened in accordancewith an increase of the tone level, tone reproduction is attained.

The scanning mode switcher 116C receives thus output halftoned tonedata, and based on a scanning mode switching signal, outputs atappropriate timing to the pulse modulator 117 only the halftoned tonedata output from one of the line buffers. The principal reason ofdisposing the two types of line buffers 116A and 116B is to deal withdifferent scanning modes for the light beam in accordance with theprinting mode as described later. The scanning mode switcher 116Cfurther ensures that the pulse modulator 117 receives the tone data atsuch timing and in such an order corresponding to each color component.In this embodiment, the line buffers 116A and 116B and the scanning modeswitcher 116C thus correspond to the “scanning mode controller” of theinvention.

In addition, the image forming apparatus having this structure iscapable of making the light beam scan in the main scanning directionback and forth. That is, the light bean can scan in both the direction(+X) and the direction (−X). The tone data constituting the 1-line imagedata are temporarily stored in a storage part (the line buffers 116A and116B) as described above, and the scanning mode switcher 116C providesthe pulse modulator 117 with the tone data at appropriate timing and ina proper order. For instance, when the direction is switched to thedirection (+X), as shown in FIG. 6A, the tone data are read out from theline buffer 116A in the order of DT1, DT2, . . . DTn and beam spots areirradiated upon the photosensitive member 2 in the first direction (+X)based on each piece of the tone data, whereby line latent images LI(+X)are formed. On the contrary, when the direction is switched to thedirection (−X), as shown in FIG. 6B, the tone data are read out from theline buffer 116B in the order of DTn, DT(n−1), . . . DT1 and beam spotsare irradiated upon the photosensitive member 2 in the second direction(−X) based on each piece of the tone data, whereby line latent imagesLI(−X) are formed. The light beam for latent image formation is thuschanged for different printing modes or different lines. Describing inmore specific details, in this embodiment, the RAM 107 temporarilystores information related to a resolution (resolution information)contained in a print command. In the event that printing at a highresolution is instructed, latent images are formed as the so-calleddouble-side scanning mode is executed which alternately repeats anoperation of making a light beam SL1 scan over the effective imageregion IR in the direction (+X) and forming latent images in theeffective image region IR and an operation of making a light beam SL2scan over the effective image region IR in the direction (−X) andforming latent images in the effective image region IR. In contrast,when printing at a low resolution is instructed, latent images areformed as the so-called single-side scanning mode is executed whichrepeats only the light beam SL1. This embodiment thus demands that thescanning mode for the light beam is switched between high-resolutionprinting and low-resolution printing in accordance with the resolutioninformation. This will be described in more detail later.

The latent image forming operation in the apparatus according to thesixth embodiment will now be described. FIG. 18 is a flow chart of thelatent image forming operation in the sixth embodiment. Upon receipt ofa print command from an external apparatus such as the host computer100, latent images are formed on the respective photosensitive membersand a color images are formed from these latent images in accordancewith the flow chart in FIG. 18. In other words, at Step S10, resolutioninformation contained in the print command is acquired (informationacquiring step). Based on the resolution information, whether the printcommand calls for printing at a high resolution or a low resolution isdetermined (Step S11).

When it is determined YES at Step S11, that is, when it is determinedprinting at a low resolution is demanded, Step S16 to Step S19 areexecuted. Through these Steps images are formed at a low resolution andtransferred onto a sheet S and printing is terminated. First, at StepS16, the apparatus is set to the single-side scanning mode (scanningmode setting step). Next, the sub-scanning line screen angles for thesingle-side scanning mode (hereinafter called single-side scanning linescreen angles) are set (Step S17). The sub-scanning line screen angleherein referred to is the angle between the respective lines of the linescreen for each color component and the sub-scanning direction Y. Thisembodiment, as shown in FIG. 19A, requires setting the single-sidesub-scanning line screen angles MYK, MMK, MCK and MKK respectively forthe color components of yellow (Y), magenta (M), cyan (C) and black (K).The single-side sub-scanning line screen angles MYK, MMK, MCK and MKKfor the respective color components are different in order to suppressdevelopment of so-called moiré fringes. Moiré fringes are known todevelop when the sub-scanning line screen angles are out of colorregistration. It is well established empirical knowledge that to makemoiré fringes less noticeable, the sub-scanning line screen angles aremost preferably shifted relatively to each other by about 30 degreesbetween two colors. Since yellow (Y) is least noticeable to human eyesas compared to the other colors, the sub-scanning line screen angles forthe other colors of magenta (M), cyan (C) and black (K) than yellow (Y)are shifted by 30 degrees from each other. In this embodiment therefore,at the stage of setting the single-side sub-scanning line screen angles,the single-side sub-scanning line screen angle MYK for yellow (Y) is setto 90 degrees, the single-side sub-scanning line screen angle MCK forcyan (C) is set to 75 degrees, the single-side sub-scanning line screenangle MMK for black (K) is set to 45 degrees, and the single-sidesub-scanning line screen angle MMK for magenta (M is set to 15 degrees.

Further, the scanning mode switching signal which corresponds to thescanning mode determined in the manner above is supplied to the scanningmode switcher 116C of the main controller 11 (Step S18). Receiving theinstruction, the scanning mode switcher 116C fixes the timing at whichand the order in which tone data should be read from the line buffer,and forms latent images line by line. In short, the tone data are readfrom the forward-direction line buffer 116A at proper timing in theforward direction (i.e., the tone data in the order of DT1, DT2, . . .DTn), and only a beam spot running in the first direction, while beingmodulated based on the respective pieces of tone data, scans over thephotosensitive member 2, whereby latent images are formed (Step S19).The so-called single-side scanning mode is executed in this fashion, andlatent images are formed at a low resolution. Thus formed latent imagesare then developed with toner, thereby forming toner images in the fourcolors. The toner images are superimposed one atop the other on theintermediate transfer belt 71, thereby forming a color image. The colorimage is thereafter transferred onto a sheet S, and printing at a lowresolution completes.

When it is determined NO at Step S11, that is, when it is determinedprinting at a high resolution is demanded, Step S12 to Step S15 areexecuted. Through these Steps which images are formed at a highresolution and transferred onto a sheet S and printing is terminated.First, at Step S12, the apparatus is set to the double-side scanningmode (scanning mode setting step). Next, the sub-scanning line screenangles for the double-side scanning mode (double-side sub-scanning linescreen angles) are set (Step S13). This embodiment, as shown in FIG.19B, requires setting the double-side sub-scanning line screen anglesMYR, MMR, MCR and MKR respectively for the color components of yellow(Y), magenta (M), cyan (C) and black (K). The double-side sub-scanningline screen angle is set smaller than the single-side scanning linescreen angle for each color, while maintaining inter-color anglesconstant. Describing in more specific details, the double-sidesub-scanning line screen angle MYR for yellow (Y) is set to 75 degrees,the double-side sub-scanning line screen angle MCR for cyan (C) is setto 30 degrees, the double-side sub-scanning line screen angle MKR forblack (K) is set to 60 degrees, and the double-side sub-scanning linescreen angle MMR for magenta (M) is set to 0 degree.

Further, the scanning mode switching signal which corresponds to thescanning mode determined in the manner above is supplied to the scanningmode switcher 116C of the main controller 11 (Step S14). Receiving theinstruction, the scanning mode switcher 116C switches the timing atwhich and the order in which tone data should be read from the linebuffer, alternately every line. Therefore, latent images are formed at ahigh resolution in the following manner. That is, an operation of makinga beam spot scan on the effective image region IR in the direction (+X)and accordingly forming latent images within the effective image regionIR and an operation of making a beam spot scan on the effective imageregion IR in the direction (−X) and accordingly forming latent imageswithin the effective image region IR are repeated alternately (StepS15). The so-called double-side scanning mode is executed in thisfashion, and latent images are formed at a high resolution. Thus formedlatent images are then developed with toner, thereby forming tonerimages in the four colors. The toner images are superimposed one atopthe other on the intermediate transfer belt 71, thereby forming a colorimage. The color image is thereafter transferred onto a sheet S, andhigh-resolution printing completes.

As described above, in the sixth embodiment, the sub-scanning linescreen angles for the double-side scanning mode are set so that theangle (the sub-scanning line screen angle) between the respective linesof a line screen and the sub scanning direction in the double-sidescanning mode is smaller than that in the single-side scanning mode.This makes it possible to suppress periodic appearances of thin sectionsand thick sections of a line latent image due to the unevenness of thescanning pitch in the sub scanning direction for the double-sidescanning mode. As a result, even in the double-side scanning mode,favorable tone reproduction is attained. The reason of this will now bedescribed in details. FIG. 20 is a drawing of the scanning pitch in thedouble-side scanning mode. FIG. 21 is an explanatory diagram regarding apattern attributable to the unevenness of the scanning pitch. In thesixth embodiment, as described above, a beam spot reciprocally scans onthe surface of the photosensitive member 2 in the main scanningdirection X while driving the surface of the photosensitive member 2 inthe sub scanning direction Y which is approximately orthogonal to themain scanning direction X. Since the scanning track of the beam spots onthe surface of the photosensitive member 2 in the double-side scanningmode is as denoted at the dotted-and-dashed line in FIG. 20, thescanning pitch in the sub scanning direction Y is not constant. Theunevenness of the scanning pitch in the sub scanning direction Y isparticularly remarkable near the both ends of the scanning track in themain scanning direction X. On the contrary, the sixth embodimentrealizes tone reproduction using a line screen which changes the linewidths of lines extending in a predetermined direction in accordancewith tones. Hence, as shown in FIG. 21, when a line latent imageextending in a predetermined direction is formed in the double-sidescanning mode for tone reproduction, because of the varying scanningpitch in the sub scanning direction Y, thin sections in which the widthof the line latent image is narrow and thick section in which the widthof the line latent image is wide are formed periodically in the linelatent image and the line latent image therefore has an unwantedpattern. The solid lines in FIG. 21 denote the scanning lines, while thecircles express beam spots which are created on the surface of thephotosensitive member. Such a pattern could obstruct favorable tonereproduction and cause image impairment.

However, in this embodiment, the sub-scanning line screen angles foreach scanning mode are set so that the angle (the sub-scanning linescreen angle) between the respective lines of the line screen and thesub scanning direction in the double-side scanning mode is smaller thanthat in the single-side scanning mode. This configuration makes itpossible in the double-side scanning mode to suppress periodicappearances of thin sections and thick sections of a line latent imagedue to the unevenness of the scanning pitch in the sub scanningdirection. FIGS. 22A, 22B, 22C and 22D are drawings showing arelationship between width of line latent image and the angle between aline screen and the sub scanning direction. In FIGS. 22A, 22B, 22C and22D, the dotted-and-dashed lines are indicative of the scanning lines,the squares denote latent images formed in areas where the scanningpitch is narrow (thin section) or wide (thick section), and the solidstraight lines denote the directions in which the respective lines ofthe line screen extend. From FIGS. 22A toward 22D, the line screen angleMI becomes smaller. In FIG. 22A where the angle is the largest, the linewidth DT in a narrow scanning-pitch area and the line width DF in a widescanning-pitch area are apparently different. However, as the linescreen angle MI becomes smaller from FIG. 22C toward FIG. 22D, thedifference between the line width DT in the narrow scanning-pitch areaand the line width DF in the wide scanning-pitch area shrinks. It isthus possible to suppress the difference between the line width DT inthe narrow scanning-pitch area and the line width DF in the widescanning-pitch area as the line screen angle MI is reduced. According tothis embodiment which requires setting the angle (the sub-scanning linescreen angles) between the respective lines of the line screen and thesub scanning direction smaller in the double-side scanning mode than inthe single-side scanning mode, it is possible in the double-sidescanning mode to suppress periodic appearances of thin sections andthick sections of a line latent image due to the unevenness of thescanning pitch in the sub scanning direction. As a result, even in thedouble-side scanning mode, favorable tone reproduction is achieved.

Further, in the sixth embodiment, the sub-scanning line screen angle MMRfor magenta (M) for the double-side scanning mode is 0 degree. In thiscondition, as shown in FIG. 22D, the line width DF and the line width DTare approximately equal to each other. That is, in this embodiment,since the angle (the double-side sub-scanning line screen angle) betweenthe respective lines of the line screen and the sub scanning directionfor the double-side scanning mode for magenta (M) is 0 degree, aninfluence of the varying scanning pitch in the sub scanning direction inthe double-side scanning mode over tone reproduction is completelysuppressed and excellent tone reproduction is realized.

Seventh Embodiment

While the sixth embodiment requires halftoning using a line screen, theseventh embodiment requires halftoning using a halftone screen. As shownin FIGS. 23A, 23B and 23C for tone reproduction, a halftone screen growshalftone dots which are spaced apart in predetermined two directions D1and D2 in accordance with tone levels. In FIGS. 23A, 23B and 23C, fromFIG. 23A toward FIG. 23C, shades become darker. The basic structure ofthe apparatus according to the seventh embodiment is the same as that ofthe apparatus according to the first embodiment, and therefore, the samestructure will be denoted at the same or corresponding reference symbolsbut will not be described in redundancy.

FIG. 24 is a flow chart of an operation of the image forming apparatusaccording to the seventh embodiment. In the seventh embodiment, uponreceipt of a print command from an external apparatus such as the hostcomputer 100, latent images are formed on the respective photosensitivemembers and a color image is formed from these latent images inaccordance with the flow chart in FIG. 24. In other words, at Step S20,resolution information contained in the print command is acquired(information acquiring step). Based on the resolution information,whether the print command calls for printing at a high resolution or alow resolution is determined (Step S21).

When it is determined YES at Step S21, that is, when it is determinedprinting at a low resolution is demanded, Step S26 to Step S29 areexecuted. Through these Steps images are formed and transferred onto asheet S and printing is terminated. First, at Step S26, the apparatus isset to the single-side scanning mode (scanning mode setting step). Next,the sub-scanning halftone screen angles for the single-side scanningmode (single-side sub-scanning halftone screen angles) are set (StepS27). FIG. 25A is a drawing of the sub-scanning halftone screen anglesin a single-side scanning mode in the seventh embodiment. FIG. 25B is adrawing of the sub-scanning halftone screen angles in a double sidescanning mode in the seventh embodiment. The sub-scanning halftonescreen angles herein referred to are angles between the sub-scanningdirection Y and the directions in which halftone dots of the halftonescreen for each color components are arranged. As described above, sincehalftone dots are arranged in the predetermined two directions in ahalftone screen, the halftone screen has two sub-scanning halftonescreen angles. Describing this in relation to black (K) for instance asa typical example for this embodiment, as shown in FIG. 25A, twosingle-side sub-scanning halftone screen angles AKK1 and AKK2 are set.Further, the scanning mode switching signal which corresponds to thescanning mode determined in the manner above is supplied to the scanningmode switcher 116C of the main controller 11 (Step S28). Receiving theinstruction, the scanning mode switcher 116C fixes the timing at whichand the order in which tone data should be read from the line buffer,and forms latent images line by line. In short, the tone data are readfrom the forward-direction line buffer 116A at proper timing in theforward direction (i.e., the tone data in the order of DT1, DT2, . . .DTn), and only a beam spot running in the first direction, while beingmodulated based on the respective pieces of tone data, scans over thephotosensitive member 2, whereby latent images are formed (Step S29).The so-called single-side scanning mode is executed in this fashion, andlatent images are formed. Thus formed latent images are then developedwith toner, thereby forming toner images in the four colors. The tonerimages are superimposed one atop the other on the intermediate transferbelt 71, thereby forming a color image. The color image is thereaftertransferred onto a sheet S, and printing of letters completes.

When it is determined NO at Step S21, that is, when it is determinedprinting at a high resolution is demanded, Step S22 to Step S25 areexecuted. Through these Steps images are formed at a high resolution andtransferred onto a sheet S and printing is terminated. First, at StepS22, the apparatus is set to the double-side scanning mode (scanningmode setting step). This is followed by setting of the angles in thedouble-side scanning mode between the sub-scanning direction Y and thedirections in which halftone dots of the halftone screen for each colorcomponents are arranged (sub-scanning halftone screen angles) (StepS23). In this embodiment, as shown in FIG. 25B, the double-sidesub-scanning halftone screen angles AKR1 and AKR2 for black (K) are set.In other words, the seventh embodiment requires, as shown in FIG. 25B,setting the double-side halftone screen angles for the double-sidescanning mode for black (K) in such a manner that the angle in thedouble-side scanning mode between the sub-scanning direction Y and oneof the two arrangement directions which is at a greater angle withrespect to the sub-scanning direction Y will be smaller than that in thesingle-side scanning mode. That is, in the single-side scanning mode,the single-side sub-scanning halftone screen angle AKK1 is set as shownin FIG. 25A, whereas in the double-side scanning mode, as shown in FIG.25B, the double-side sub-scanning halftone screen angle AKR1, which isthe larger one among the two double-side sub-scanning halftone screenangles AKR1 and AKR2, is set to be smaller than the single-sidesub-scanning halftone screen angle AKK1 which is the larger one amongthe two single-side sub-scanning halftone screen angles AKK1 and AKK2.The dashed lines in FIG. 25B are indicative of, for comparison, thedirections in which halftone dots are arranged in the single-sidescanning mode. In the seventh embodiment, the arrangement directionwhich is at a smaller angle with respect to the sub-scanning direction Yis common between the single-side scanning mode and the double-sidescanning mode. In short, the single-side sub-scanning halftone screenangle AKK2 is the same as the double-side sub-scanning halftone screenangle AKR2. In addition, the seventh embodiment demands changing thesub-scanning halftone screen angles between the single-side scanningmode and the double-side scanning mode only as for black K.

Further, the scanning mode switching signal which corresponds to thescanning mode determined in the manner above is supplied to the scanningmode switcher 116C of the main controller 11 (Step S24). Receiving theinstruction, the scanning mode switcher 116C switches the timing atwhich and the order in which tone data should be read from the linebuffer, alternately every line. Therefore, latent images are formed inthe following manner. That is, an operation of making a beam spot scanon the effective image region IR in the direction (+X) and accordinglyforming latent images within the effective image region IR and anoperation of making a beam spot scan on the effective image region IR inthe direction (−X) and accordingly forming latent images within theeffective image region IR are repeated alternately (Step S25). Theso-called double-side scanning mode is executed in this fashion, andlatent images are formed. Thus formed latent images are then developedwith toner, thereby forming toner images in the four colors. The tonerimages are superimposed one atop the other on the intermediate transferbelt 71, thereby forming a color image. The color image is thereaftertransferred onto a sheet S, and photo printing completes.

As described above, the seventh embodiment requires setting thesub-scanning halftone screen angles so that the angle in the double-sidescanning mode between the sub-scanning direction Y and one of the twoarrangement directions which is at a greater angle with respect to thesub scanning direction Y will be smaller than in the single-sidescanning mode. In other words, in the double-side scanning mode, thelarger double-side sub-scanning halftone screen angle of the twodouble-side sub-scanning halftone screen angles is set to be smallerthan the larger single-side sub-scanning halftone screen angle of thetwo single-side sub-scanning halftone screen angles. It is thereforepossible in the double-side scanning mode to suppress the appearance ofan unwanted pattern which is attributable to the unevenness of thescanning pitch in the sub-scanning direction Y. This realizes excellenttone reproduction even in the double-side scanning mode. The reason ofthis will now be described in details. In this embodiment, as describedabove, a beam spot reciprocally scans on the surface of thephotosensitive member 2 in the main scanning direction while driving thesurface of the photosensitive member 2 in the sub scanning direction Ywhich is approximately orthogonal to the main scanning direction X.Since the scanning track of the beam spots on the surface of thephotosensitive member 2 in the double-side scanning mode is as denotedat the dotted-and-dashed line in FIG. 20, the scanning pitch in the subscanning direction Y is not constant. The unevenness of the scanningpitch in the sub scanning direction Y is particularly remarkable nearthe both ends of the scanning track in the main scanning direction X. Onthe contrary, the seventh embodiment realizes tone reproduction using ahalftone screen which grows halftone dots which are spaced apart in thetwo arrangement directions. Hence in the event that halftones are formedin the predetermined directions for tone reproduction in the double-sidescanning mode as shown in FIG. 26A, due to the varying scanning pitch inthe sub scanning direction, the widths DT and DF of the halftone dots inthe vertical direction relative to the arrangement directions of thehalftone dots change periodically, thereby creating an unnecessarypattern. FIGS. 26A, 26B, 26C and 26D are drawings which show arelationship between the widths of the halftone dots and the angle ofthe arrangement directions of the halftone dots with respect to the subscanning direction. From FIG. 26A to FIG. 26D, the dotted-and-dashedlines are indicative of the scanning lines, the squares denote halftonedots formed in areas where the scanning pitch is narrow or wide, and thesolid lines are indicative of the arrangement directions of the halftonedots.

The periodical changes of the halftone dot widths described above aredependent upon the angle between the arrangement directions of thehalftone dots and the sub scanning direction. This will now be describedwith reference to FIGS. 26A, 26B, 26C and 26D. From FIG. 26A to FIG.26D, the angle between the arrangement directions of the halftone dotsand the sub scanning direction Y (sub-scanning halftone screen angle AI)becomes smaller. In FIG. 26A where the angle is the largest, thehalftone dot width DT in a narrow scanning-pitch area and the halftonedot width DF in a wide scanning-pitch area are significantly different.However, as the angle between the arrangement directions of the halftonedots and the sub scanning direction (sub-scanning halftone screen angleAI) becomes smaller from FIG. 26C toward FIG. 26D, the differencebetween the halftone dot width DT in the narrow scanning-pitch area andthe halftone dot width DF in the wide scanning-pitch area shrinks. Thus,the smaller the sub-scanning halftone screen angle AI is, the smallerthe difference between the halftone dot width DT in the narrowscanning-pitch area and the halftone dot width DF in the widescanning-pitch area is. Noting this, this embodiment requires settingthe sub-scanning halftone screen angle so that the angle between the subscanning direction Y and one of the two arrangement directions presentwithin the halftone screen which forms a greater angle with the subscanning direction Y and in which the halftone dot width varies moregreatly will be smaller in the double-side scanning mode than in thesingle-side scanning mode. In other words, the larger sub-scanninghalftone screen angle of the two sub-scanning halftone screen angles inthe double-side scanning mode is set to be smaller than that in thesingle-side scanning mode. It is therefore possible to suppress in thedouble-side scanning mode the appearance of an unwanted pattern which isattributable to the unevenness of the scanning pitch in the sub-scanningdirection. This realizes excellent tone reproduction even in thedouble-side scanning mode.

Eighth Embodiment

FIG. 27 is a block diagram of signal processing in the eighthembodiment. As for the eighth embodiment, a difference from the earlierembodiments described above will be mainly described. The commonportions will be denoted at corresponding reference symbols but will notbe described. In the illustrated image forming apparatus, upon receiptof an image signal from an external apparatus such as the host computer100, the main controller 11 performs predetermined signal processing ofthe image signal. The main controller 11 comprises functional blockssuch as the color converter 114, the image processor 115, the two typesof line buffers 116A and 116B, the scanning mode switcher 116C, thepulse modulator 117.

As described earlier, in addition to the CPU 101, the ROM 106, the RAM107 and the exposure controller 102 shown in FIG. 2, the enginecontroller 10 is equipped with the tone characteristic detector 123which detects a tone characteristic which expresses the γ-characteristicof the engine part EG based on the result of detection yielded by thedensity sensor 76. The eighth embodiment hence permits calculating thetone reproduction characteristic of the apparatus as it is duringexecution of each scanning mode while the engine controller 10 and themain controller 11 control the respective portions of the apparatus inthe manner described later. The engine controller 10 and the maincontroller 11 thus function as the “single-side characteristicidentifier” and the “double-side characteristic identifier” of theinvention.

In the main controller 11 supplied with the image signal from the hostcomputer 100, the color converter 114 converts RGB tone data intocorresponding CMYK tone data, the RGB tone data representing therespective tone levels of RGB components of each pixel in an imagecorresponding to the image signal, the CMYK tone data representing therespective tone levels of CMYK components corresponding to the RGBcomponents. In the color converter 114, the input RGB tone data comprise8 bits per color component for each pixel (or representing 256 tonelevels), for example, whereas the output CMYK tone data similarlycomprise 8 bits per color component for each pixel (or representing 256tone levels). The CMYK tone data outputted from the color converter 114are inputted to the image processor 115.

For each color component, the image processor 115 halftones tone datafor each pixel fed from the color converter 114. This halftoning may beforming one halftone dot using multiple pixels and growing the size ofthe halftone dot in accordance with the tone level representing the tonedata to thereby reproduce a tone. A method of creating halftone dotswhich grow in accordance with the tone level of tone data may be adither method, an error diffusion method, etc. This embodiment uses adither method for halftoning. FIG. 28 is an explanatory diagram of adither method. According to a dither method, the tone level of inputtone data is compared with the threshold value of each element of athreshold matrix MTX. One element of the threshold matrix corresponds toone pixel. When the tone level of input tone data is larger than thethreshold value of each element, the value at a location correspondingto this element is “1” and a latent image is formed at a location on thesurface of the photosensitive member corresponding to this location. Onthe contrary, when the tone level of input tone data is smaller than thethreshold value of each element, the value at a location correspondingto this element is “0” and no latent image is formed at a location onthe surface of the photosensitive member corresponding to this location.Where a dither method is used, the tone level of input tone data iscompared with the threshold value of each element of the thresholdmatrix, thereby obtaining halftoned tone data. In the example in FIG.28, the threshold matrix MTX can reproduce 16 different tone levels isused and the tone level of the tone data is 4. However, the thresholdmatrix MTX is not limited to the example shown in FIG. 28 but insteadmay be one which is capable of reproducing more tone levels. Thearrangement of threshold values in the threshold matrix MTX is notlimited to that shown in FIG. 28 either, but may be modified dependingupon the necessity.

As described above, this embodiment uses a dither method for halftoning.In other words, comparing tone data received from the color converter114 with the threshold matrix stored in a matrix storage part 110A whichis a non-volatile memory, the image processor 115 converts the tone datainto halftoned tone data. Further, in an attempt to maintain theγ-characteristic of the image forming apparatus always ideal, thisembodiment requires executing tone control processing of updating, atpredetermined timing, the content of the threshold matrix stored in thematrix storage part 110A based on the actually measured density of animage.

During the tone control processing, for each toner color, the enginepart EG forms on the intermediate transfer belt 71 tone-correcting tonepatch images which are prepared in advance for measurement of theγ-characteristic, the density sensor 76 reads the image densities of therespective tonal patch images, and based on a signal from the densitysensor 76, the tone characteristic detector 123 generates a tonecharacteristic (the γ-characteristic of the engine part EG) whichcorrelates the tone levels of the respective tone patch images with thedetected image densities and outputs the tone characteristic to athreshold value conversion table calculator 110B of the main controller11. Based on the tone characteristic fed from the tone characteristicdetector 123, the threshold value conversion table calculator 110Bcompensates the measured tone characteristic of the engine part EG andcalculates a threshold value conversion table which is for obtaining anideal tone characteristic, and the content of the threshold matrixstored in the matrix storage part 110A is updated based on the yieldedcalculation result. The image forming apparatus is thus capable offorming images in a stable quality despite any variation of theγ-characteristic of the apparatus, a change with time, etc.

The two types of line buffers 116A and 116B receive the halftoned tonedata obtained in the manner described above. The operations and thestructure of the line buffers 116A and 116B are as described earlier.

The scanning mode switcher 116C receives the halftoned tone data thusoutput. At proper timing, the scanning mode switcher 116C outputs to thepulse modulator 117 only the halftoned tone data output from one linebuffer based on the scanning mode switching signal. The scanning modeswitcher 116C supplies to the pulse modulator 117 the tone data at suchtiming and in such an order corresponding to each color component. Inthis embodiment the line buffers 116A and 116B and the scanning modeswitcher 116C thus correspond to the “scanning mode controller” of theinvention. In this embodiment, the matrix storage part 110A stores asingle-side threshold matrix 1101 and a double-side threshold matrix1102 which respectively correspond to the respective scanning modes, andin each scanning mode, executes the tone control processing mentionedabove and accordingly updates the single-side threshold matrix 1101 andthe double-side threshold matrix 1102, which will be described next.

FIG. 29 is a flow chart of the tone control processing in thesingle-side scanning mode (single-side characteristic detecting step)according to the eighth embodiment. First, the apparatus is set to thesingle-side scanning mode (Step S11). Next, the scanning mode switchingsignal which corresponds to the scanning mode determined in the mannerabove is supplied to the matrix storage part 110A (Step S12). Inresponse to this, the matrix storage part 110A outputs the single-sidethreshold matrix 1101 to the image processor 115. Tonal patch images arethen formed on the intermediate transfer belt 71 (Step S13). These tonalpatch images are formed on the intermediate transfer belt in accordancewith a tone generation pattern which is for arranging plural tonerimages having predetermined different tone levels from each other in thedirection in which the intermediate transfer belt 71 is driven. Theplural toner images are arranged so that the tone levels of therespective toner images are progressively lower with respect to the beltdriving direction. While toner images may be formed spanning all tonelevels from the maximum tone level to the minimum tone level, in theeighth embodiment, toner images having only predetermined tone levelsare formed. At Step S12, the matrix storage part 110A is set so as tooutput the single-side threshold matrix 1101 to the image processor 115.Hence, it is the single-side threshold matrix 1101 stored in the matrixstorage part 110A that is used to form tonal patch images. Next, thedensity sensor 76 detects the densities of the plural toner images whichare at different tone levels of thus formed patch images (Step S14). Thetone characteristic detector 123 then generates a tone characteristic asthat denoted at the solid line in FIG. 30 which correlates the tonelevels to the detected image densities (Step S15). FIG. 30 is anexplanatory diagram on the tone characteristic. From the tonecharacteristic generated in this fashion, the threshold value conversiontable calculator 110B compiles a single-side threshold value conversiontable which makes the image density change linearly as the tone levelchanges. And the threshold values of the single-side threshold matrixare corrected with the compiled single-side threshold value conversiontable (Step S16). In short, the threshold values in the single-sidethreshold matrix are corrected so that the tone characteristic denotedat the solid line in FIG. 30 will become linear as denoted at the dashedline in FIG. 30. Those threshold values corresponding to tone levels atwhich no toner image is formed as the tonal patch images are calculatedby linearly interpolating the single-side threshold value conversiontable. The content of the threshold matrix 1101 stored in the matrixstorage part 110A is then updated to the content of the correctedsingle-side threshold matrix (Step S17). The tone correction processingin the single-side scanning mode thus corresponds to the “single-sidecharacteristic detecting step” of the invention, and the updatedsingle-side threshold matrix corresponds to the “tone reproductioncharacteristic” for the single-side scanning mode of the invention.

FIG. 31 is a flow chart of the tone control processing in thedouble-side scanning mode (double-side characteristic detecting step)according to the eighth embodiment. First, the apparatus is set to thedouble-side scanning mode (Step S21). Next, the scanning mode switchingsignal which corresponds to the scanning mode determined in the mannerabove is supplied to the matrix storage part 110A (Step S22). Inresponse to this, the matrix storage part 110A outputs the double-sidethreshold matrix 1102 to the image processor 115. Tonal patch images arethen formed on the intermediate transfer belt 71 (Step S23). These tonalpatch images are formed on the intermediate transfer belt in accordancewith a tone generation pattern which is for arranging plural tonerimages having predetermined different tone levels from each other in thedirection in which the intermediate transfer belt 71 is driven. Theplural toner images are arranged so that the tone levels of therespective toner images are progressively lower with respect to the beltdriving direction. While toner images may be formed spanning all tonelevels from the maximum tone level to the minimum tone level, in theeighth embodiment, toner images having only predetermined tone levelsare formed. At Step S22, the matrix storage part 110A is set so as tooutput the double-side threshold matrix 1102 to the image processor 115.Hence, it is the double-side threshold matrix 1102 stored in the matrixstorage part 110A that is used to form tonal patch images. In the eighthembodiment, the double-side threshold matrix and the single-sidethreshold matrix are in the relationship which is shown in FIGS. 32A and32B. FIG. 32A is a drawing which shows the threshold value matrices inthe single-side scanning. FIG. 32B is a drawing which shows thethreshold value matrices in the double-side scanning mode. That is,since the scanning pitch in the sub scanning direction Y in thedouble-side scanning mode is roughly half that in the single-sidescanning mode, one element in the single-side threshold matrix iscorrelated with two side-by-side elements in the sub scanning directionY in the double-side threshold matrix so that the threshold value atthese elements is the same. Thus, in the eighth embodiment, the tonegeneration pattern for tonal patch images in the double-side scanningmode is common to the tone generation pattern for tonal patch images inthe single-side scanning mode.

The density sensor 76 detects the densities of plural toner images whichare at different tone levels of thus formed patch images as tonal patchimages (Step S24). The tone characteristic detector 123 then generates atone characteristic as that denoted at the solid line in FIG. 30 whichcorrelates the tone levels to the detected image densities (Step S25).From the tone characteristic generated in this fashion, the thresholdvalue conversion table calculator 110B compiles a double-side thresholdvalue conversion table which makes the image density change linearly asthe tone level changes. And the threshold values of the double-sidethreshold matrix are corrected with the compiled double-side thresholdvalue conversion table (Step S26). In short, the threshold values in thedouble-side threshold matrix are corrected so that the tonecharacteristic denoted at the solid line in FIG. 30 will become linearas denoted at the dashed line in FIG. 30. Those threshold valuescorresponding to tone levels at which no toner image is formed as thetonal patch images are calculated by linearly interpolating thedouble-side threshold value conversion table. The content of thedouble-side threshold matrix 1102 stored in the matrix storage part 110Ais then updated to the content of the corrected double-side thresholdmatrix (Step S27). The tone correction processing in the double-sidescanning mode thus corresponds to the “double-side characteristicdetecting step” of the invention, and the updated double-side thresholdmatrix 1102 corresponds to the “tone reproduction characteristic” forthe double-side scanning mode of the invention.

As described above, in the eighth embodiment, it is possible to switchbetween the single-side scanning mode and the double-side scanning mode.Such an image forming apparatus is capable of performing the latentimage forming operation while switching the scanning mode in accordancewith the printing mode. According to the eighth embodiment therefore,the scanning mode is switched depending upon a resolution. In short,when a resolution is not asked, a latent image is formed in thesingle-side scanning mode which uses a wide scanning pitch in the subscanning direction, whereas when a resolution is demanded, a latentimage is formed in the double-side scanning mode which uses a narrowscanning pitch in the sub scanning direction. The latent image formingoperation according to the eighth embodiment will now be described.

FIG. 33 is a flow chart of the latent image forming operation in theimage forming apparatus. Upon receipt of a print command from anexternal apparatus such as the host computer 100, latent images areformed on the respective photosensitive members and a color image isformed from these latent images in accordance with the flow chart inFIG. 33. In other words, at Step S30, resolution information containedin the print command is acquired. Based on the resolution information,whether the print command calls for printing at a high resolution or alow resolution is determined (Step S31).

When it is determined YES at Step S31, that is, when it is determinedprinting at a low resolution is demanded, Step S36 to Step S39 areexecuted. Through these Steps images are formed at a low resolution andtransferred onto a sheet S and printing is terminated. First, at StepS36, the apparatus is set to the single-side scanning mode. Next, thescanning mode switching signal which corresponds to the scanning modedetermined in the manner above is supplied to the matrix storage part110A (Step S37). In response to this, the matrix storage part 110Aoutputs the single-side threshold matrix 1101 to the image processor115. The image processor 115 generates halftoned tone data using thesingle-side threshold matrix 1101, and output the halftoned tone data tothe corresponding line buffer. The scanning mode switching signal whichcorresponds to the scanning mode determined in the manner above issupplied further to the scanning mode switcher 116C of the maincontroller 11 (Step S38). Receiving the instruction, the scanning modeswitcher 116C fixes the timing at which and the order in which tone datashould be read from the line buffer, and forms latent images line byline. In short, the tone data are read from the forward-direction linebuffer 116A at proper timing in the forward direction (i.e., the tonedata in the order of DT1, DT2, . . . DTn), and only a beam spot runningin the first direction, while being modulated based on the respectivepieces of tone data, scans over the photosensitive member 2, wherebylatent images are formed (Step S39). The so-called single-side scanningmode is executed in this fashion, and latent images are formed at a lowresolution. Thus formed latent images are then developed with toner,thereby forming toner images in the four colors. The toner images aresuperimposed one atop the other on the intermediate transfer belt 71,thereby forming a color image. The color image is thereafter transferredonto a sheet S, and low-resolution printing completes.

When it is determined NO at Step S31, that is, when it is determinedprinting at a high resolution is demanded, Step S32 to Step S35 areexecuted. Through these Steps images are formed at a high resolution andtransferred onto a sheet S and printing is terminated. First, at StepS32, the apparatus is set to the double-side scanning mode. Next, thescanning mode switching signal which corresponds to the scanning modedetermined in the manner above is supplied to the matrix storage part110A (Step S33). In response to this, the matrix storage part 110Aoutputs the double-side threshold matrix 1102 to the image processor115. The image processor 115 generates halftoned tone data using thedouble-side threshold matrix 1102, and output the halftoned tone data tothe corresponding line buffer. The scanning mode switching signal whichcorresponds to the scanning mode determined in the manner above issupplied further to the scanning mode switcher 116C of the maincontroller 11 (Step S34). Receiving the instruction, the scanning modeswitcher 116C switches the timing at which and the order in which tonedata should be read from the corresponding line buffer, alternatelyevery line. Therefore, latent images are formed at a high resolution inthe following manner. That is, an operation of making a beam spot scanon the effective image region IR in the direction (+X) and accordinglyforming latent images within the effective image region IR and anoperation of making a beam spot scan on the effective image region IR inthe direction (−X) and accordingly forming latent images within theeffective image region IR are repeated alternately (Step S35). Theso-called double-side scanning mode is executed in this fashion, andlatent images are formed at a high resolution. Thus formed latent imagesare then developed with toner, thereby forming toner images in the fourcolors. The toner images are superimposed one atop the other on theintermediate transfer belt 71, thereby forming a color image. The colorimage is thereafter transferred onto a sheet S, and high-resolutionprinting completes.

As described with reference to FIG. 20, by means of the structure aboveaccording to the invention, a beam spot reciprocally scans on thesurface of the photosensitive member 2 in the main scanning direction Xwhile driving the surface of the photosensitive member 2 in the subscanning direction Y which is approximately orthogonal to the mainscanning direction X. Since the scanning track of the beam spots on thesurface of the photosensitive member 2 in the double-side scanning mode,is as denoted at the dotted-and-dashed line in FIG. 20, the scanningpitch in the sub scanning direction Y is not constant. Due to this, whenone wishes to form a latent image using a beam spot which scans on thesurface of the photosensitive member 2 in the double-side scanning mode,the degree of overlapping of beam spots in the sub scanning direction Ycould vary because the scanning pitch in the sub scanning direction Y isnot constant. In other words, beam spot overlaps in the sub scanningdirection Y are large in areas where the scanning pitch in the subscanning direction Y is narrow, while beam spots overlaps in the subscanning direction Y are small in areas where the scanning pitch in thesub scanning direction Y is wide. Hence, in the double-side scanningmode, a color could have light shades and dark shades due to the varyingscanning pitch in the sub scanning direction Y.

In contrast, according to the eighth embodiment, toner images are formedas tonal patch images through execution of the single-side scanning modeand the tone reproduction characteristic during the single-side scanningmode is controlled based on the detected densities of the toner images,and in the double-side scanning mode, toner images are formed as tonalpatch images and the tone reproduction characteristic during thedouble-side scanning mode is controlled based on the detected densitiesof the toner images. In short, in each one of the single-side scanningmode and the double-side scanning mode, toner images serving as tonalpatch images are formed and the tone reproduction characteristic of theapparatus for each scanning mode is optimized based on the detecteddensities of the toner images. Hence, regardless of whether the tonereproduction characteristic changes between the double-side scanningmode and the single-side scanning mode, it is possible to realizefavorable tone reproduction in either scanning mode.

Further, in the eighth embodiment, during the tone control processing,toner images are formed as tonal patch images based on the same tonegeneration pattern in both the single-side scanning mode and thedouble-side scanning mode. This eliminates the necessity of providing atone generation pattern for each scanning mode and simplifies thestructure.

Ninth Embodiment

The ninth embodiment requires scanning the surface of the photosensitivemember 2 (surface to be scanned) with the light beam, using the scanninglens 66 which exhibits an arc-sign theta lens characteristic (FIG. 4). Abeam spot created by the scanning lens 66 on the surface of thephotosensitive member 2 from the light beam which is deflected by thedeflection mirror surface 651 which oscillates in sine motions asdescribed earlier scans over the surface of the photosensitive member 2at an equal speed in the main scanning direction X. A line-shaped latentimage extending in the main scanning direction X is consequently formedon an effective scan region ESR on the photosensitive member 2. In theninth embodiment, the scan region SR which can be scanned using thedeflector 65 is wider than the effective scan region ESR as shown inFIG. 4. In addition, the effective scan region ESR is locatedapproximately at the center of the scan region SR and is approximatelysymmetric with respect to the optical axis.

In the ninth embodiment as well, the light beam can scan back and forthin the main scanning direction X. That is, the light beam can scan inboth the direction (+X) and the direction (−X). In the ninth embodimenttherefore, as the light beam scans reciprocally in the main scanningdirection X, line latent images LI(+X) and line latent images LI(−X) areformed alternately in the sub scanning direction Y on the surface of thephotosensitive member 2. A characteristic aspect of the ninth embodimentalone will be described below, while common portions will not bedescribed.

In the ninth embodiment, the beam spot diameter Wb in the sub scanningdirection Y of a beam spot formed on the surface of the photosensitivemember 2 is equal to or larger than the maximum scanning pitch in thesub scanning direction Y in “end portions” of the effective scan regionESR but is equal to smaller than double the minimum scanning pitch. Bymeans of this structure, the beam spot diameter Wb is equal to or largerthan the maximum scanning pitch in the sub scanning direction Y withinthe effective scan region ESR but is equal to smaller than double theminimum scanning pitch. The reason of this will now be described indetails. FIG. 34 is a drawing which shows a relationship between thebeam spot diameter and the scanning pitch. In the ninth embodiment, abeam spot having a constant beam spot diameter reciprocally scans at theconstant speed in the main scanning direction X within the effectivescan region ESR which is provided on the surface of the photosensitivemember 2. Meanwhile, the surface of the photosensitive member 2 isdriven in the sub scanning direction Y which is approximately orthogonalto the main scanning direction X. Hence, the track of the scanning linesof the beam spot on the surface of the photosensitive member 2 (surfaceto be scanned) is zigzag as denoted at the dotted-and-dashed lines inFIG. 34. It is therefore in the “end portions” of the effective scanregion ESR that the scanning pitch in the sub scanning direction Ybecomes the maximum or the minimum within the effective scan region ESR.Hence, the beam spot diameter Wb in the sub scanning direction Y of thebeam spot, being set to be equal to or larger than the maximum scanningpitch in the sub scanning direction Y in the “end portions” of theeffective scan region ESR but equal to smaller than double the minimumscanning pitch, is equal to or larger than the maximum scanning pitch inthe sub scanning direction Y within the effective scan region ESR butequal to smaller than double the minimum scanning pitch.

As described above, in the structure according to the ninth embodiment,the beam spot diameter Wb in the sub scanning direction Y of a beam spotformed on the surface of the photosensitive member 2 is equal to orlarger than the maximum scanning pitch in the sub scanning direction Yin the end portions of the effective scan region ESR. The beam spotdiameter Wb in the sub scanning direction Y of a beam spot formed on thesurface of the photosensitive member 2 is therefore equal to or largerthan the maximum scanning pitch in the sub scanning direction Y withinthe effective scan region ESR. Hence, beam spots are connected with eachother in the sub scanning direction Y in an area where the scanningpitch is narrow of course and also in an area where the scanning pitchis wide, which attains favorable two-dimensional scanning on the surfaceof the photosensitive member 2. In consequence, image impairmentdescribed later is prevented and an excellent image is formed. Thereason will now be described in detail.

The ninth embodiment requires making a beam spot reciprocally scan thesurface of the photosensitive member 2 at a constant speed in the mainscanning direction X while driving the surface of the photosensitivemember 2 in the sub scanning direction Y which is approximatelyorthogonal to the main scanning direction X. The track of the scanninglines of the beam spot on the surface of the photosensitive member 2(surface to be scanned) is therefore zigzag as denoted at thedotted-and-dashed lines in FIG. 34. The scanning pitch in the subscanning direction Y within the effective scan region ESR is thus notconstant as denoted at the reference symbols PT1 through PT3. In thisstructure, when one wishes to form a line image extending in the subscanning direction Y for instance, such image impairment could occurthat beam spots fail to overlap each other in the sub scanning directionY and the line image is cut in an area where the scanning pitch in thesub scanning direction Y is wide. In contrast, this embodiment requiressetting the beam spot diameter Wb in the sub scanning direction Y of abeam spot to be equal to or larger than the maximum scanning pitchwithin the effective scan region ESR which has a predetermined width inthe main scanning direction X on the surface of the photosensitivemember. Hence, beam spots are connected with each other in the subscanning direction Y in an area where the scanning pitch is narrow ofcourse and also in an area where the scanning pitch is wide, whichattains favorable two-dimensional scanning. It is thus possible toprevent the image impairment mentioned above of a disconnected lineimage and instead to form an excellent image.

The ninth embodiment further requires setting the beam spot diameter Wbin the sub scanning direction Y of a beam spot formed on the surface ofthe photosensitive member 2 to be equal to or smaller than double theminimum scanning pitch in the sub scanning direction Y in the endportions of the effective scan region ESR. The beam spot diameter Wb inthe sub scanning direction Y of a beam spot formed on the surface of thephotosensitive member 2 is therefore equal to or smaller than double theminimum scanning pitch in the sub scanning direction Y within theeffective scan region ESR. Spots therefore do not overlap excessivelyeach other in the sub scanning direction Y even in an area where thescanning pitch is narrow within the effective scan region ESR, therebyrealizing excellent two-dimensional scanning. It is thus possible toprevent image impairment described later and instead to form anexcellent image. The reason of this will now be described in detail.

As shown in FIG. 34, the scanning pitch in the sub scanning direction Yis not constant in the ninth embodiment. Hence, when the beam spotdiameter in the sub scanning direction Y of a spot is too large,excessive beam spot overlapping occurs in the sub scanning direction Yin an area where the scanning pitch in the sub scanning direction Y isnarrow. Latent images formed by those beam spots having such a beam spotdiameter as well excessively overlap each other in the sub scanningdirection Y, which leads to image impairment that toner adheresexcessively to the overlaps of the latent images during development ofthe latent images, and a line image becomes too thick or shades becometoo dark. The ninth embodiment however ensures that the beam spotdiameter in the sub scanning direction Y of a beam spot is equal to orsmaller than double the minimum scanning pitch in the sub scanningdirection Y. In other words, as the upper limit value is set for thebeam spot diameter in the sub scanning direction Y, excessiveoverlapping of beam spots in the sub scanning direction Y is prevented.This prevents excessive overlapping of latent images which are formedthrough irradiation with the beam spots, and hence, avoid the imageimpairment mentioned above which is caused by excessive adhesion oftoner. It is therefore possible to form an excellent image.

Others

The invention is not limited to the embodiments described above but maybe modified in various manners in addition to the embodiments above, tothe extent not deviating from the object of the invention. For instance,although the first and the second embodiments described earlier requireswitching between the double-side scanning mode and the single-sidescanning mode based on a demanded resolution or whether tonereproduction is necessary, the criterion as for switching of thescanning mode is not limited to these. The invention is generallyapplicable to any image forming apparatus which is structured to becapable of switching between the double-side scanning mode and thesingle-side scanning mode.

Further, the scanning mode controller may control the light-sourcedriver and adjust the timing of emission from the light source, forselective switching between the single-side scanning mode and thedouble-side scanning mode.

Further, although the embodiments described above require controllingthe latent image forming operation based on the horizontal synchronizingsignal detected on the opposite side to the drive motor MT in the mainscanning direction X, the number, the arrangement and the like of thesensors are not limited to this. For example, at the both ends of thescanning route of the scanning light beam, return mirrors 69 a and 69 bmay guide the scanning light beam to horizontal synchronization sensors60A and 60B, as shown in FIG. 35. In the illustrated apparatus, when thehorizontal synchronization sensors 60A and 60B receive the scanninglight beam and the scanning light beam moves passed the sensor locations(the oscillation angle θs), the horizontal synchronization sensors 60Aand 60B output signals. The latent image forming operation may becontrolled based on the output signals from the horizontalsynchronization sensors 60A and 60B. In addition, since it is possibleto obtain detection signals at the both ends in the main scanningdirection X, the latent image forming operation may be controlled basedon the detection signal which is output from the upstream-side sensor(detector) in the scanning direction of the light beam. Alternatively,the scanning light beam may be detected using one horizontalsynchronization sensor 60C and return mirrors 69 c through 69 e as shownin FIG. 36.

Further, although the sixth embodiment described above requires settingthe sub-scanning line screen angles such that the angles between the subscanning direction Y and the respective lines of the line screen(sub-scanning line screen angles) for the double-side scanning mode willbe smaller than those in the single-side scanning as for all colorcomponents of yellow (Y), magenta (M), cyan (C) and black (K), theseangles may be set as for only selected color components. For instance,the sub-scanning line screen angle may be set only as for such a colorcomponent for which the sub-scanning line screen angle is large.Alternatively, the sub-scanning line screen angle may be set only as fora color component for which an unwanted pattern created due to theuneven scanning pitch is noticeable.

Further, although the seventh embodiment described above requiressetting the sub-scanning halftone screen angle only for black (K) suchthat the angle between the sub scanning direction and one of the twoarrangement directions present within the halftone screen which is at agreater angle with respect to the sub scanning direction will be smallerin the double-side scanning mode than in the single-side scanning mode,the invention is not applicable only to black (K) but is applicable alsoto the other color components of yellow (Y), magenta (M) and cyan (C).For instance, the invention may be applied to all color components ofyellow (Y), magenta (M), cyan (C) and black (K).

Further, although the eighth embodiment described above requireshalftoning by a dither method, halftoning is not limited to a dithermethod. An error diffusion method for example may be used instead.

Further, although the sixth through the eighths embodiments describedabove require switching between the double-side scanning mode and thesingle-side scanning mode depending upon a demanded resolution, thecriterion as for switching of the scanning mode is not limited to this.The invention is applicable generally to any image forming apparatuswhich is structured to be capable of switching between the double-sidescanning mode and the single-side scanning mode.

Further, although the effective scan region ESR is located approximatelyat the center of the scan region SR, according to the ninth embodimentdescribed above, the invention is not limited only to this application.For example, the center line of the effective scan region ESR may beshifted from that of the scan region SR in the main scanning direction Xas described later in relation to the fifth embodiment below.

Further, although the first through the eighths embodiments describedabove use only the light beam SL1 which scans in the direction (+X) inthe single-side scanning mode, the light beam SL2 which scans in thedirection (−X) may be used. The requirement here is merely to ensurethat the light beam scans only one way in the first direction (+X) orthe second direction (−X).

Further, although the embodiments described above are directed to theapplication of the invention to a color printer of the so-called tandemtype, the invention is not limited only to this application. Forexample, the invention is applicable also to a printer of the so-called4-cycle type or a monochrome printer which prints only in single color.

Further, although the embodiments described above are directed to theapplication of the invention to an image forming apparatus in which acolor image is formed temporarily on an intermediate transfer mediumsuch as an intermediate transfer belt and thereafter transferred onto asheet S, the invention is applicable also to an apparatus in which tonerimages are superimposed one atop the other directly on a sheet to form acolor image.

Further, although the embodiments described above require manufacturingthe oscillating deflection mirror surface 651 using a micromachiningtechnique, a method of forming the deflection mirror surface is notlimited to this. The invention is generally applicable to any imageforming apparatus in which a deflection mirror surface which oscillatesdeflects a light beam and makes the light beam scan on a latent imagecarrier.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiment, as well asother embodiments of the invention, will become apparent to personsskilled in the art upon reference to the description of the invention.It is therefore contemplated that the appended claims will cover anysuch modifications or embodiments as fall within the true scope of theinvention.

The invention will be understood more readily with reference to thefollowing examples; however these examples are intended to illustratethe invention and not to be construed to limit the scope of theinvention.

The first and the second examples described below both use a methodusually referred to as “a dither method” for tone reproduction. Beforedescribing each example, tone reproduction using a dither method will bedescribed with reference to FIGS. 37A, 37B and 37C which show examples.According to a dither method, a threshold matrix as that shown in FIG.37A are changed to the arrangement shown in FIG. 37B for instance fortone reproduction. Each element in the threshold matrix corresponds toone pixel, and has a predetermined threshold value. The density valueexpressed by corrected tone data is compared to the threshold value ofeach element of the threshold matrix, and whether to form latent imagesat pixels corresponding to these elements is determined. Describing inmore specific details, in the event that the density value expressed bycorrected tone data is 4 for example, latent images are formed at pixelscorresponding to those elements having the threshold values of 4 orsmaller. Meanwhile, when the density value expressed by corrected tonedata is 8, latent images are formed at pixels corresponding to thoseelements having the threshold values of 8 or smaller. The tonereproduced as the number of pixels at which latent images will be formedis changed in accordance with the density value.

First Example

The first example uses a line screen for tone reproduction and requiresswitching the sub-scanning line screen angle for black K between 45degrees which is the sub-scanning line screen angle for the single-sidescanning mode and 26.6 degrees which is the sub-scanning line screenangle for the double-side scanning mode. Means which realizes a linescreen having such sub-scanning line screen angles will now be describedwith reference to FIGS. 37 and 38.

FIGS. 37A, 37B and 37C are explanatory diagrams regarding means whichrealizes the sub-scanning line screen angles for the single-sidescanning mode according to the first example. The arrangement in thetone table shown in FIG. 37A in this example is as shown in FIG. 37B,the size of one pixel is 42.3 μm in both the main scanning direction andthe sub scanning direction. This structure realizes a line screen whichthickens a line extending at 45 degrees with respect to the sub scanningdirection in accordance with an increase of a density value as shown inFIG. 37C.

FIGS. 38A, 38B and 38C are explanatory diagrams regarding means whichrealizes the sub-scanning line screen angle for the double-side scanningmode according to the first example. The arrangement in the tone tableshown in FIG. 38A in this example is as shown in FIG. 38B, the size ofone pixel is 42.3 μm in both the main scanning direction, but in the subscanning direction, 21.2 μm which is roughly half the size for thesingle-side scanning mode, considering switching to the double-sidescanning mode. This structure realizes a line screen which thickens aline extending at 26.6 degrees with respect to the sub scanningdirection in accordance with an increase of a density value as shown inFIG. 38C.

As described above, in the first example the sub-scanning line screenangle for the single-side scanning mode is 45 degrees but is 26.6degrees for the double-side scanning mode for black (K). The firstexample thus requires that for black (K), in the double-side scanningmode, the angle between the sub scanning direction and the respectivelines of the line screen (sub-scanning line screen angles) is smallerthan that in the single-side scanning mode. This makes it possible, inthe double-side scanning mode, to suppress periodic appearances of thinsections and thick sections of a line latent image due to the unevennessof the scanning pitch in the sub scanning direction. It is thereforepossible to realize excellent tone reproduction in the double-sidescanning mode as well.

Second Example

The second example uses a halftone screen for tone reproduction, andrequires, for yellow (Y), switching between the sub-scanning halftonescreen angles of 14.04 degrees and 75.7 degrees for the single-sidescanning mode and the sub-scanning halftone screen angles of 18.43degrees and 71.57 degrees for the double-side scanning mode. Means whichrealizes a halftone screen having such sub-scanning halftone screenangles will now be described with reference to FIGS. 39A, 39B, 39C, 40A,40B and 40C.

FIGS. 39A, 39B and 39C are explanatory diagrams regarding means whichrealizes the sub-scanning halftone screen angles for the single-sidescanning mode according to the second example. The arrangement in thetone table shown in FIG. 39A in this example is as shown in FIG. 39B,the size of one pixel is 42.3 μm in both the main scanning direction andthe sub scanning direction. This structure realizes a halftone screenwhich grows the halftone dots in the directions at 14.04 degrees and75.7 degrees with respect to the sub scanning direction in accordancewith an increase of a density value as shown in FIG. 39C.

FIGS. 40A, 40B and 40C are explanatory diagrams regarding means whichrealizes the sub-scanning halftone screen angles for the double-sidescanning mode according to the second example. The arrangement in thetone table shown in FIG. 40A in this example is as shown in FIG. 40B,the size of one pixel is 42.3 μm in both the main scanning direction,but in the sub scanning direction, 21.2 μm which is roughly half thesize for the single-side scanning mode, considering switching to thedouble-side scanning mode. This structure realizes a halftone screenwhich grows the halftone dots arranged in the directions at 18.43degrees and 71.57 degrees with respect to the sub scanning direction inaccordance with an increase of a density value as shown in FIG. 40C.

As described above, in the second example, as for yellow (Y), the anglebetween the sub scanning direction and one of the two halftone dotarrangement directions which is at a greater angle with respect to thesub scanning direction is 75.7 degrees in the single-side scanning modebut is 71.57 degrees in the double-side scanning mode. The secondexample thus requires that for yellow Y, the angle between the subscanning direction and one of the two halftone dot arrangementdirections which is at a greater angle with respect to the sub scanningdirection is smaller in the double-side scanning mode than in thesingle-side scanning mode. In short, the larger sub-scanning halftonescreen angle among the two sub-scanning halftone screen angles which thehalftone screen has is set to be smaller in the double-side scanningmode than in the single-side scanning mode. This suppresses creation ofan unwanted pattern attributable to periodic changes of the halftone dotwidth. It is therefore possible to realize excellent tone reproductionin the double-side scanning mode as well.

Third Example

FIG. 41A shows a single-side threshold matrix which is used in the thirdexample. In the third example, using this single-side threshold matrix,tonal patch images having the tone levels of 2, 4, 6, 8, 10, 12, 14 and16 as those shown in FIG. 41C are formed on the intermediate transferbelt through execution of the single-side scanning mode. Table 1 is asingle-side threshold value conversion table compiled from the detecteddensities of these tonal patch images, following a similar procedure tothat of the tone control processing described earlier in relation to theprecedent examples. As for tone levels 1, 3, 5, 7, 9, 11, 13 and 15 atwhich no patch image is formed, Table 1 is linearly interpolated,thereby obtaining a single-side threshold matrix as that shown in FIG.42A. To form latent images in the single-side scanning mode, halftoningis performed using the single-side threshold matrix shown in FIG. 42A.

TABLE 1 threshold value before threshold value after conversionconversion 0 0 2 1.7 4 3.3 6 5.7 8 8.7 10 12 12 14 14 15.3 16 16

FIG. 41B shows a double-side threshold matrix which is used in the thirdexample. Since the scanning pitch in the double-side scanning mode isroughly half that in the single-side scanning mode according to thethird example, noting this, one element in the single-side thresholdmatrix is correlated with two elements in the double-side thresholdmatrix which are side by side so that the threshold value at theseelements is the same. Hence, when tonal patch images having the tonelevels of 2, 4, 6, 8, 10, 12, 14 and 16 are formed on the intermediatetransfer belt using the double-side threshold matrix through executionof the double-side scanning mode, tonal patch images as those shown inFIG. 41C are obtained which have the same tone generation pattern asthat of tonal patch images which are obtained through execution of thesingle-side scanning mode. Table 2 is a double-side threshold valueconversion table compiled from the detected densities of these tonalpatch images, following a similar procedure to that of the tone controlprocessing described earlier in relation to the precedent examples. Asfor tone levels 1, 3, 5, 7, 9, 11, 13 and 15 at which no patch image isformed, Table 2 is linearly interpolated, thereby obtaining adouble-side threshold matrix as that shown in FIG. 42B. To form latentimages in the double-side scanning mode, halftoning is performed usingthe double-side threshold matrix shown in FIG. 42B.

TABLE 2 threshold value before threshold value after conversionconversion 0 0 2 3.3 4 6 6 8.3 8 10.3 10 12.3 12 14 14 15.3 16 16

As described above, in this example, in the single-side scanning mode,toner images are formed as tonal patch images and the tone reproductioncharacteristic during the single-side scanning mode is controlled basedon the detected densities of the toner images, and in the double-sidescanning mode, toner images are formed as tonal patch images and thetone reproduction characteristic during the double-side scanning mode iscontrolled based on the detected densities of the toner images. Inshort, in each one of the single-side scanning mode and the double-sidescanning mode, toner images serving as tonal patch images are formed andthe tone reproduction characteristic of the apparatus is optimized ineach scanning mode based on the detected densities of the toner images.When one wishes to form latent images through execution of thesingle-side scanning mode or the double-side scanning mode, it ispossible to form latent images using the tone reproductioncharacteristic which is optimized for each scanning mode. Regardless ofwhether the tone reproduction characteristic changes between thedouble-side scanning mode and the single-side scanning mode therefore,it is possible to realize excellent tone reproduction in each scanningmode.

Further, in this example, during the tone control processing, tonerimages are formed as tonal patch images based on the same tonegeneration pattern in both the single-side scanning mode and thedouble-side scanning mode. This eliminates the necessity of providing atone generation pattern for each scanning mode and simplifies thestructure.

Fourth Example

FIG. 43 is an explanatory diagram of the fourth example. In the fourthexample, the effective scan region ESR accounts for 50% of the scanregion SR, and the center line of the effective scan region ESR and thatof the scan region SR are the same. In addition, since a resolution inthe sub scanning direction Y is 1200 dpi, the scanning pitch in the subscanning direction Y on the center line of the effective scan region ESRis about 21.2 μm. At this stage, the maximum scanning pitch in the subscanning direction Y in the end portions of the effective scan regionESR is PTmax1=32.8 μm. Noting this, in the fourth example, the beam spotdiameter in the sub scanning direction Y of a beam spot is 33 μm whichexceeds the maximum scanning pitch PTmax1 in the sub scanning directionY. The adjustment of the beam spot diameter connects beam spots witheach other in the sub scanning direction Y in an area where the scanningpitch is short of course and also in an area where the scanning pitch islong, which attains favorable two-dimensional scanning. It is thuspossible to form an excellent image.

Fifth Example

FIG. 44 is an explanatory diagram of the fifth example. In the fifthexample, the effective scan region ESR accounts for 60% of the scanregion SR, and the center line of the effective scan region ESR isshifted from that of the scan region SR in the first direction (+X) by5% in terms of the proportion to the scan region SR. In addition, sincea resolution in the sub scanning direction Y is 2400 dpi, the scanningpitch in the sub scanning direction Y on the center line of theeffective scan region ESR is about 10.6 μm. At this stage, the maximumscanning pitch in the sub scanning direction Y in the end portions ofthe effective scan region ESR is PTmax2=18.02 μm. Noting this, in thefifth example, the beam spot diameter in the sub scanning direction Y ofa beam spot is 20 μm which exceeds the maximum scanning pitch PTmax2 inthe sub scanning direction Y. The adjustment of the beam spot diameterconnects beam spots with each other in the sub scanning direction Y inan area where the scanning pitch is short of course and also in an areawhere the scanning pitch is long, which attains favorabletwo-dimensional scanning. It is thus possible to form an excellentimage.

Sixth Example

FIG. 45 is an explanatory diagram of the sixth example. In the sixthexample, the effective scan region ESR accounts for 20% of the scanregion SR, and the center line of the scan region SR and that of theeffective scan region ESR are the same. In addition, since a resolutionin the sub scanning direction Y is 2400 dpi, the scanning pitch in thesub scanning direction Y on the center line of the effective scan regionESR is about 10.6 μm. At this stage, the maximum scanning pitch in thesub scanning direction Y in the end portions of the effective scanregion ESR is PTmax3=12.72 μm. Meanwhile, the minimum scanning pitch inthe sub scanning direction Y on the center line of the effective scanregion ESR is PTmin3=8.48 μm. Noting this, in the sixth example, thebeam spot diameter in the sub scanning direction Y of a beam spot is 15μm which is equal to or larger than the maximum scanning pitch PTmax3 inthe sub scanning direction Y but equal to or smaller than double theminimum scanning pitch PTmin3 in the sub scanning direction Y. With thebeam spot diameter adjusted to be equal to or larger than the maximumscanning pitch PTmax3 in the sub scanning direction Y, beam spots areconnected with each other in the sub scanning direction Y in an areawhere the scanning pitch is short of course and also in an area wherethe scanning pitch is long, which attains favorable two-dimensionalscanning. Further, with the beam spot diameter adjusted to be equal toor smaller than double the minimum scanning pitch PTmin3 in the subscanning direction Y, excessive overlapping of beam spots in the subscanning direction Y is prevented. This prevents excessive overlappingof latent images which are formed through irradiation with the beamspots, and hence, avoid the image impairment mentioned above which iscaused by excessive adhesion of toner. It is therefore possible to forman excellent image.

1. A light scanning apparatus comprising: a light source which emits alight beam; a deflector which has a deflection mirror oscillating abouta drive axis approximately orthogonal to a main scanning direction, thedeflection mirror reflecting the light beam emitted from the lightsource so as to scan the light beam reciprocally in the main scanningdirection; and an imaging optical system which focuses the light beamdeflected by the deflector on a surface to be scanned so as to form abeam spot on the surface, the surface being driven in a sub scanningdirection approximately orthogonal to the main scanning direction andincluding an effective scan region spanning across a predetermined widthin the main scanning direction, wherein a diameter of the beam spot inthe sub scanning direction is equal to or larger than a maximum scanningpitch in the sub scanning direction within the effective scan region,and the scanning pitch in the sub scanning direction is not constant. 2.A light scanning apparatus of claim 1, wherein the diameter of the beamspot in the sub scanning direction is equal to or smaller than doublethe minimum scanning pitch in the sub scanning direction within theeffective scan region.
 3. A light scanning apparatus of claim 1, whereinthe diameter of the beam spot in the sub scanning direction is equal toor larger than the maximum scanning pitch in the sub scanning directionin end portions of the effective scan region.
 4. A light scanningapparatus of claim 1, wherein the diameter of the beam spot in the subscanning direction is equal to or larger than the maximum scanning pitchin the sub scanning direction in end portions of the effective scanregion, but is equal to or smaller than double the minimum scanningpitch in the sub scanning direction in end portions of the effectivescan region.
 5. An image forming apparatus, comprising: a latent imagecarrier whose surface includes an effective scan region spanning acrossa predetermined width in a main scanning direction and is driven in asub scanning direction approximately orthogonal to the main scanningdirection; a light source which emits a light beam; a deflector whichhas a deflection mirror oscillating about a drive axis approximatelyorthogonal to a main scanning direction, the deflection mirrorreflecting the light beam emitted from the light source so as to scanthe light beam reciprocally in a main scanning direction; and an imagingoptical system which focuses the light beam deflected by the deflectoron the surface of the latent image carrier so as to form a beam spot onthe surface, wherein a diameter of the beam spot in the sub scanningdirection is equal to or larger than the maximum scanning pitch in thesub scanning direction within the effective scan region, and thescanning pitch in the sub scanning direction is not constant.